Wario's never been a darker character. He's alway had goofiness baked into his character. During his conception Kiyotake even compared Mario and Wario to Popeye and Bluto. He's a cartoon character, always has been. The kind who will toss buckets...
The thing about Wario is that yeah, he is a selfish jerk, but he is a selfish jerk in a cartoony setting that's meant to be fun. His actions are meant to be enjoyable to watch, so of course there are lines with him.
WarioWare tends to show that...