Because the chances of someone who disagrees with progressives being a murderous lunatic is much higher than someone who agrees with progressives being a murderous lunatic.
Empathy. Being able to see circumstances from a different perspective and appreciating how that makes the other person feel. And then not wanting to hurt the other people. Empathy, not selfishness.
Because the people who would disagree with a bad person are generally good enough people that they wouldn't kill someone. If a bad person disagrees with a good person, all bets are off.