Ska hasn't been around since the 50s. What everyone calls "first wave" was just reggae anyway. 2nd wave didn't even hit till waht, the late 70s/early 80s? Thats when ska becamerecognizable as ska. Metal is from about the same era. Not even black metal is about hate, its about castles and dragons and ridiculous stuff.
I wouldn't even go as far as to say ska/punk is all about unity. Though there are a percentage bands that have a song or two about that, its not so much the main theme. Like the Business and Dropkick Murphys are about getting drunk and cracking skulls, and so are The Casualties to some extent (when you can actually understand what they're saying lol). Even some ska bands like Leftover Crack/Crackrocksteady 7 are pretty gruesome.
You're preaching to the choir here..I've seen pretty much every punk/ska band that was ever even decent except the dead kennedys. Now that I'm on the topic, I've even met a decent percentage of them.