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.
Aside from being borne out by modern and distant history, it makes sense that a group of people who literally don't care about the needs of their neighbors are more receptive to murdering them.
Good, intelligent people are progressive because they are good and intelligent. Evil, unhinged, stupid, and uneducated people are likely to be conservative because they are stupid, uneducated, evil, or crazy.
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.