I still don't know what the difference between those perfect modals, especially between "might", "should" and "must".
- It was wrong of Glen to be so rude. He ______ (not behave) that way.
- Something ______ (happen) to Steve. He always calls when he's late.
Especially for sentence 1, why don't we use must? In my opinion, I think everyone who became rude is actually wrong. It's like "should" = something that you have to do/something right, but if you ignore, it doesn't matter. While "must" = Something that you have to do, and if you ignore it, you will face the consequences.
Why do I use "must"? Because, I think on this problem "It was wrong of Glen to be so rude.", we can see the consequences. Think about it, when someone being rude, no one want to see him nor talk to him. There is a consequences.