Also you literally could not be a cop, if you cannot kill someone in self-defense then you cannot be expected to kill in the defense of others. It is not a pretty part of the job but every so often is a necessary one, if cops refuse to kill, if they all only carried one-shot tazers then they would die a lot more often than they do now and a lot more innocent people would die with them.

Things like this do happen:
[url=http://en.wikipedia.org/wiki/North_Hollywood_shootout]

It is miracle there were no fatalities except for the shooters and it was catastrophic enough that the police lacked rifles or other weapons with the necessary capability to defeat the body armor of the shooters.