From Princess Mononoke:
アラァ！！ やっぱり いい男じゃない
Araa!! Yappari ii otoko ja nai
Oh!! I knew it, you are a good-looking man!


ja nai is negative, so why doesn't it mean, "I knew you weren't a good man."?

Thanks!

It's not really a negative sentence. The じゃない is just looking for confirmation.

Think of it like this : I knew it, you are a good guy aren't you?

Ah that makes sense. Thank you!

I guess it's a bit like ne or desho?