Neither. It's just informal. You can call a friend dude: "Hey dude let's go have a beer" or use it to address someone you aren't friendly with: "Hey dude, get out of my face or I'll kick your ass." It's used for males, but I've heard some females use it for females too in a joking way.

Is "dude" a bad word? No. Does it sound silly when adults use it? Yes.

Depends on how old you are.

ちがい、よい！it is "slang." Non-formal. Young people use it sometimes. it is not bad, but be careful! you would not want to call your boss "dude!"

It's not bad, just very informal. You can use it amongst your friends if you're younger, but definitely don't call your boss "dude." If you're forty and calling people "dude," it could sound weird; it would make your English sound a little immature. It's not a swear word or something that's necessarily insulting, though. It's very North American English, I think; I've never heard a British person call someone "dude." And people from some areas of the U.S., at least, use it more than others.