I don't really see slaad as evil, just more of a survival of the fittest sort of thing. I think really the problem is that module writers just make slaad aggresive because they're chaotic, which is just lazy. Personally, I see slaad as sort of thrying to constantly prove which one is the greatest. The lower slaad constantly fight, becuase they're too stupid to prove themselves any other way.
I see it more like this; red and blue slaad's aren't very smart. They have an Inteligence and Wisdom of 6; that makes them dumber than orcs, and ogres, which also have a 6 Int, have a 10 Wis. I think they're also more strongly driven by their instinct to eat and mate because of their lack of intellectual prowess; there's less conscious thought to reign instinctive impulses. Even if they do understand other creatures, they're chaotic and pretty stupid, so I see it as a VERY low attention span. You start trying to explain advanced philosophy to a red or blue slaad, he'll just get bored and hungry.
Green slaads are different. They got a 10 Int and Wis, which is the same as the average human. They also seek power through magical might, so they're not just a bunch of bullies. Green slaads are described as arrogant, not necessarily belligerent. They're not murderous, but they also don't like to share, either. A green isn't goning to go around randomly killing people, he's not going to gain knowledge and power that way. They might go around killing wizards and taking their stuff, but they might also make trades with wizards to gain power. They'll be heavy-handed about it to be sure, but not necessarily murderous.
