Rocket is the only genuinely malicious group without a silly master plan, they kill, steal, disrupt and destroy. They know they are evil, they don't believe they are right in what they do, nor have any special ideology to follow, simply the usual criminal persuits of power and wealth, because that's what works. Notice how Rocket is the only team still active after everything the player does in any given game? Yeah.
Although, as I am what we could call a first generation Pokemon fan, nostalgia is super effective (I can't believe I just said that...), so I'm probably just being a biased old fart.