Should behavior and misbehavior be synonyms? I can't imagine someone asking about behavior because of their concern over it being good. Behavior has more associated questions, so having misbehavior point to behavior seems the way to go.
Misbehavior is always behavior, but not necessarily the other way around. I've used the behavior tag on questions like How to inspire more creativity in play? where the child wasn't behaving badly, just not in a way I understood. This is reflected in the tag wiki:
Questions about why children act the way they do. See the tag discipline about enforcing your rules.
Part of enforcing your rules is understanding why children are acting the way they do, so there's a lot of overlap and the boundary isn't always strongly enforced. Looking at the misbehavior questions, I think making it a synonym of discipline might be more appropriate.