Criminals are neither tough nor brave. Those are characteristics of good people. This is probably why handguns are so effective against criminals.
There are a lot of dead individuals, both LEO and non-LEO, who had the same idea.
Criminals are people, with as many different motivations for action as your average group. Some are cowards. Others are bullies. Others are looking at a third strike and want to kill witnesses to stay out of prison. Others don't care if they are alive or dead. Others may cut and run, but won't when they see that one of their pack is standing their ground.
If you rely on a criminal to be a coward, you are underestimating your opponent. That is one of the surest ways of achieving failure.