Which of these words is most synonymous with "normal"?

At first I almost chose "common" but then I thought about it longer and chose "understandable". True, a lot of the times whats considered the "normal" thing to do is usually commonly done amongst ourselves, but there are things that are normal that aren't commonly done. To best explain what I mean, I will use an example because I find myself struggling to find the right words. There have been many serial killers throughout history, some more well known than others. Its fairly common to hear about them, but what they do is not normal at all. Why is it not normal, because what they do is not "understandable". Theres no real understandable reason as to why they act this way. I mean sure if you ask them why they did it they may say "the voices told me to do it" but it doesn't make sense, its not understandable. They didnt do it in self defense, or to defend their family, they just did it because they heard "voices". Ugh, my example still doesnt explain what I'm trying to say! So frustrating, so I will just leave it at that. Hopefully someone will understand what I'm trying to say lol