I believe you are under a mistaken assumption. It's never ok to kill the suspect. Still, one can be justified in using lethal force. It's a very subtle distinction. You never shoot to kill. That's simply murder. You shoot to stop the attack -- even if that's very likely to result in the death of the person you're shooting at. It can be justifiable to shoot somebody running at you with a deadly weapon. Shutting down the circulation is a fairly quick and reliable way to stop somebody from taking actions that threaten your life. But, once they stop running at you and crumple to the ground, you have to stop shooting. If you continue shooting, that's murder. If somebody turns to run, a civilian must stop shooting or it's murder. (In certain circumstances, the police can and should continue shooting.) Again, it's subtle.

So, it's not ok to intentionally kill the suspect, but it can be ok to use deadly force. I'll assume you're really asking why deadly force might be acceptable. A pipe in the hand is a lethal weapon. If a police officer is duty-bound to apprehend a person who is using lethal force against him, it would be insane to use anything less than lethal force in response. If an officer attempts to use less effective non-lethal means, but fails and is killed, then he is not only dead, but also has failed in his duty to protect the public just as if he had run away.

It's important to remember that these situations can happen extremely fast, under enormous stress. A routine encounter can turn deadly serious in a fraction of a second. The whole thing can be over in under three seconds. There is no time to think or reason. To think is to hesitate, and to hesitate is to die. You must have a trained reflex: if they use deadly force, then I shoot at center-of-mass. Seeing a pipe in the hand of an approaching person might be the trigger for that reflex. It's not quite so simple, though. It's best to prime the reflex by thinking it through before-hand. Police officers must constantly maintain situational awareness, thinking through possible scenarios and priming their reflexes based on who is around and how they're acting. It's a very difficult and dangerous job.