If he wants to save Rapunzel, he should not be too emotional that he is unaware of Mother Gothel. As a result, he does not help free Rapunzel from the chain. Just fortunately, he cuts Rapunzel's hair (make her hair ugly) so Rapunzel will not be bound to Mother Gothel.

mmmmmmm what a good question but i think that he didn't know about the plan don't u think and gothel was smarter than him but he suffer just for her and i think that is cute

Well, he never actually met the woman before he stabbed her and he didn't know much about her, either, so it's not like he could have known.

Well he didn't know that she hid behind him and plunged the dagger into his stomach and techincally he hadn't a choice but to chop Rapunzel's hair off because he loves her.

I think maybe he was a little too naive about going about there, but he didn't know Mother Gothel and he wasn't aware of how psycho and dangerous she actually was.

It wasn't a very careful thing to do!

Yes, he should have been aware, but I could see how and why he fell for Mother Gothel's trap.