Yep

Bringing it back to reality-ish - pick a spot? now.

Let's consider, did all these people you'd want to go back in time to assassinate have anything in common? Did they have any traits that could be identified in living people?

My guess is the keystone trait we're looking for is a distinct lack of empathy. Combine a lack of empathy with certain other traits and you get some real nasty combinations.

Example

Ambition with empathy = good person doing big good things

Ambition without empathy = bad person doing big bad things