For example, Christianity spread through Europe by the sword- they viewed pagans (and anyone of a different religion) as automatically evil, and felt justified in killing them if they refused to convert. Since people's definitions of what is "evil" are often self-serving, to open the door to slaughtering people based on your own perceptions would be disastrous.
This is a GREAT question! Kind of like the who would you save in a house fire....I always thought if someone actually crossed my path in life that was evil I would recognize the Evil but as I grow up I find that it is very difficult to know another persons heart/soul and evil people are actually very good at deception.
Well that is a very broad ? ... If I knew that the person was guilty of a crime that deserves death then I could pull the switch/inject the poison. Could I hunt them down and kill them out right? I don't know. Depends on the actual crime and proof.
The Bible tells us at Exodus 20:13 that we "should not murder." Nowhere does it say that we have the right to do away with evildoers. Rather, it says Jehovah God has the only authority to do so. (Deuteronomy 32:35)