Even when he is playing around, Eddie MacKenzie — a/k/a Eddie Mac — can be a terrifying prospect. "You ever do that again," he says, leaning in, "I’m gonna make you disappear." He leans in further, his voice barely rising above a murmur. "That’s how easy it is." By now, Eddie Mac is so close you can see the individual fibers in his scally cap. You imagine that you can hear the muscles pinging under his tight black T-shirt. You notice that his cap is made by Nike. You start wondering how much hair he has under there. Anything to avoid looking into those eyes.

Eddie Mac’s eyes are wide, deep-set, and bright blue. They are not cold and hard like you’d expect. They are not filled with predatory disinterest or murderous rage. Eddie Mac’s eyes are inquisitive, mischievous. Like a child’s. Because of this, and knowing what you know about the man, those eyes can send your stomach spiraling, particularly when he tells you one of his funny stories. "Once I smashed a guy’s leg in half, beating his fucking brains out," he says. "I got this raging hard-on, man, I’m about to ejaculate. I grab my friend who was with me and say, ‘Dude, I’m hard as a rock!’ We were both laughing."

Street Soldier: My Life As an Enforcer for Whitey Bulger and the Boston Irish Mob (Steerforth Press), Edward MacKenzie Jr.’s recently published and much-ballyhooed memoir, is full of stories like this. The book recounts MacKenzie’s years in the thick of the lawlessness that gripped South Boston in the 1970s, ’80s, and ’90s. Though Bulger features quite heavily in the story, the real star here is MacKenzie, 45, who seems to delight in putting himself in a very, very bad light. And yet, as a tell-all, the book actually tells a little less than MacKenzie would have liked, partly because his attorney took out the anecdotes that could have led to criminal charges, and partly because some of it was, well, a little much.

"There was some really, uh, how shall we say, um, deviant behavior that made the publisher sick," MacKenzie says, sipping on a soft drink in the South Shore Plaza. "He said people will be repulsed when they read it, you can’t put this in there, you can’t put that in there. Ross [Muscato, one of the book’s co-authors] would be like, ‘What the fuck!’ Phyllis [Karas, the other co-author] would be, ‘Oy vey!’ I’d say, ‘Don’t cut this shit out. Put it all in there.’ But this is the literary world. They were like, ‘Nooo!’ "

All the same, there is plenty in Street Soldier to give the reader’s hackles a workout.

Eddie Mac’s tale begins in the summer of 1958, the year of his birth. "I came," he writes in the book’s opening pages, "from shit stock." In many ways, MacKenzie’s story is depressingly familiar: his mother, Charlotte, had seven kids, three of them with MacKenzie’s father, Edward Sr., the rest with individuals unknown. She was 21 when she had MacKenzie; his father was 16. Predictably, perhaps, the parents showed less interest in raising their children than in conceiving them, and in 1963, MacKenzie and his siblings were committed to the care of the Department of Public Welfare.

Bounced among foster homes and state-run facilities, MacKenzie didn’t have an easy time of it — he describes beatings and deprivations, administered by various Dickensian characters. In 1967, when he was nine years old, MacKenzie was raped. "From behind, he put his arm around my waist and pulled me toward him," MacKenzie writes of the tutor who befriended and then assaulted him. "The pain was like nothing I’d experienced before."

This incident, recounted early in the book, serves as a backdrop for everything that follows — the petty crime, the sexual excess, the drug dealing, the intimidation, the terrible violence. "At the age of nine, when I was bent over and sodomized, that started my life, that planted the predatory seed," MacKenzie says now in a matter-of-fact tone. "That’s when I decided not to be prey any more and become the predator. You make a decision: am I going to get eaten or am I going to eat?"

The rest of Street Soldier documents, in graphic detail, Eddie Mac’s involvement in the kind of crime the authorities like to call "heinous." "I was as vicious as they come," he writes, "a monster." Worse yet, Eddie Mac was a monster with a couple of kickboxing championships under his belt — a skill that, coupled with his love for violence, made him one of the most feared street thugs in an area where street thugs were in rich abundance. "No one had a chance," he writes, describing a bar brawl he particularly enjoyed. "Just running and swinging, we smashed everyone, women as well as men. We went right down the bar, busting skulls, jaws, and ribs. What a rush."

Eventually, Eddie Mac’s fearsome reputation drew the attention of mobster James J. "Whitey" Bulger, who took the hotheaded street fighter and molded him into a hotheaded henchman — a role MacKenzie performed with exemplary dedication between 1985 and 1990, when he was handed an indictment for cocaine trafficking.

The case against him was damning, but MacKenzie is nothing if not a survivor. Like many indicted mobsters of the time, he avoided serving prison time by cooperating with the feds. He is quick to point out, however, that he only informed on a Colombian cocaine cartel — a violation of the Southie street code that, as far as MacKenzie is concerned, was nowhere near as egregious as the one committed by his former boss, the FBI informant. "I never informed on my neighborhood," he says, "not on my friends, not even on Whitey Bulger. So if people ask me if I’m a rat, I go, ‘Yeah, absolutely I’m a rat. But there’s a big difference between this rat and the guy who you and me and everyone else looked up to. He ratted out everyone to get his freedom.’ "

This is about as close as MacKenzie comes to sounding contrite about his past. Though the original title for his memoir was The Redemption of Eddie Mac, there is very little in the way of out-and-out remorse about it. Street Soldier, certainly, is a confessional in only the loosest sense. Throughout the book, MacKenzie recounts the blood he spilled, the bones he broke, and the lives he shattered with an attention to detail that verges on the celebratory. One passage in particular bears repeating in full:

Once my victim was sprawled across the sidewalk, out cold, I’d go over his body with as much precision as a surgeon in the operating room. First, I’d probe with my feet, kicking each rib, feeling a high every time I heard one snap beneath my sneaker. Then, with one or two swift heel kicks, I’d attack the leg bones. Then I’d lift up the arms, one at a time, and smash, there went the ribs underneath, broken as easily as sticks underfoot. I worked methodically until the body had been pummeled to my satisfaction. Sometimes I even used my teeth, biting off an ear and spitting it back at the body: a farewell present for my victim when he opened his bloodied, swollen eyes. Once, I went for a finger instead and actually swallowed it. Made me a little sick, but nowhere as pained as the poor bastard that had to go through the rest of his life with nine fingers.

In person, Eddie is not much of a self-editor: if he wants to tell you about the time he shot his wad all over a two-way mirror, he will — unapologetically, maybe even gleefully. His book is the same way, which can make for an unsettling read. While true-crime buffs may relish tales of violence and depravity, we also expect a certain amount of distance between us and the horrors on the page — either by way of an impartial (or disapproving) observer, or by way of a first-person narrative that contains contrition, or at least some sort of transformation. You want the author to take a step back every now and then, to recoil from his own viciousness, to say, "That wasn’t me." You certainly don’t want him to be wallowing in the stuff.

MacKenzie, meanwhile, scoffs at these sorts of "leafy suburb" sentiments. "Just because you don’t like the messenger, don’t discount the message," he says. "I don’t give a shit about who thinks this is fucked up. People want you to say, ‘Oh, woe is me!’ Fuck that. I’m not sorry. I’m not purged. You go back to the psyche of the predator. Does the lion feel bad for ripping the baby zebra apart? I’m trying to be as honest as I can here. I’m not going to be a fake for anybody."

The fact is, MacKenzie doesn’t want readers to be sympathetic toward him. "I was a scumbag," he says. "I was not a good person." Though there are a couple of politically sensitive moments in the book — one a gay-bashing incident, the other an instance of violent racism — that are recounted with something approaching embarrassment, MacKenzie insists that this tone was the result of editorial twiddling. "That wasn’t me," he says.