They're trained to use their gun with lethal intent in situations where their lives, or those of civilians, are in danger. Failure to "end" the threat in such a situation could prove fatal to the officers or bystanders.
The abuse of power is the issue (as you suggested), "tests" can't really "weed out" the idiots. No one is going to admit that they have deep rooted psychological issues/inferiority complexes during a psych eval. We know this all too well, it's a ubiquitous trope; those who seek power/authority over others are, in general, the least deserving. There are some good cops, just men doing their job, the others, not so much.
Give power/authority (and a gun) to people less than righteous and constantly expose them to people that have severe issues with authority (especially authority granted by a badge and a gun). It's a recipe for disaster.