As he walked toward the driver’s side of the 1997 Infiniti SUV he had pulled over early Sunday morning, Auburn police Officer Ron Tarentino Jr. was approaching the most vulnerable, dangerous moment and space in a cop’s universe: death’s door.

He was approaching that moment, when a cop isn’t quite sure what he’s got, and that space, where there’s a criminal waiting who has already made up his mind that he is not going back to prison. It is a space and moment when a desperate criminal holds a distinct advantage over a cop who is desperate to know what’s in the mind of the guy sitting in the driver’s seat.

Death’s door can appear on a nondescript street in a small Central Massachusetts town as naturally as it does on the H block of Boston’s toughest neighborhood, where Boston police Officer John Moynihan approached death’s door last year.

Moynihan got lucky. The bullet that Antonio West fired into his face from the driver’s seat of the car Moynihan had approached didn’t kill him. Moynihan’s partners were able to quickly pursue West and shoot him dead after he turned and shot at them.

Tarentino wasn’t as lucky and didn’t have that kind of backup, so it took police the better part of a day to track Jorge Zambrano to a duplex in Oxford. Like West, somebody who went to court and jail with a routine that others save for work, Zambrano was, I’m guessing, determined to not go back to jail and was committed, I’m sure, to taking as many cops with him as possible. He had to know he was going to die. Every action he took ensured it.

Zambrano could have surrendered. Instead, he lay in wait and ambushed the cops who were searching for him. He managed to wound one of them, a state trooper. But, as is usually the case in these situations, the cops are better shots.

In the coming days, Tarentino will be deeply mourned, and his fellow officers who, with inspiring selflessness, hunted down his killer will be appropriately commended. Tarentino died doing what hundreds of police officers across this commonwealth — what thousands across this nation — do every day, and that’s approach death’s door, not quite sure what awaits them. It is a singular act of bravery that cops perform so often that it is taken for granted.

But beyond honoring Tarentino’s service and sacrifice, beyond saluting the extraordinary courage of the cops who at tremendous risk to themselves systematically searched that duplex knowing that Jorge Zambrano had already killed a police officer and was hell-bent on killing more, we need to know why Zambrano was in a position to do this.

He had a long record and was sent to prison at least twice, including a seven-year sentence for among other things cocaine trafficking, attacking a cop, and carrying a silencer. But he appears to have been back on the street and in trouble by 2014. He was arrested again at least a couple of times this year, as recently as last week. It looks like he caught breaks, made bail, and somehow avoided getting put away for violating probation. There’s a lot to answer for here.

Maybe it’s more than the obvious, that he just didn’t want to go back to jail. But a guy can only catch so many breaks before he finds himself sitting behind death’s door, holding a gun.

Recently, I spent some time talking to members of the Boston police gang unit. Every one of them complained about guys they’ve arrested with guns catching breaks, getting out on bail.

Bail is primarily about ensuring somebody will show up in court. But a judge is allowed to take into account dangerousness. And there’s nothing more dangerous than that space, that moment, when a guy who is facing charges that can send him back to jail sits there behind death’s door, sizing up both his chances and the cop drawing nearer in the sideview mirror.

Kevin Cullen is a Globe columnist. He can be reached at cullen@globe.com. Follow him on Twitter@GlobeCullen.