What I have always liked about this bumper sticker is the attribution to Jesus. A while back, I decided that it was fairly pointless to try to convince people that killing other people is wrong. Instead, I just refer them to Jesus' words. If they want to argue the point, I suggest they argue it with Jesus. Good luck with that.
Maybe I'm getting old, but my energy level for trying to convince people how to believe is wearing thin. I AM willing, however, and eager to spread the word about Jesus and what he said and stands for. There is room for interpretation for much of what he is quoted as having said. But the passage about loving one's enemies is clear. It's not merely a suggestion. The bumpersticker effectively reminds the reader of that, and politely suggests one ramification of putting it into practice- the ways of the decedent, pagan, secular world notwithstanding.