By the power of truth, I, while living, have conquered the universe.
Gravemind isn't truly 'evil' though, from his perspective he's doing everyone a favour by assimilating them: "Arrogant creature! Your deaths will be instantaneous while we must suffer the progress of infinitude!" ~ Gravemind, Halo 2, High Charity
He's also willing to comprimise and work with his 'enemies' because they share a mutual goal, he considers the Flood to be a reprieve for his victims as it unites them as a 'perfect' form of life - free of civil conflict and chaos, the Flood is completely at one with itself.
With Truth, he's a madman who knows that all life in the galaxy is going to be wiped out but is going to do it anyway because he's off his rocker. He had no problem causing an internal war within the ranks of the Brutes and Elites just to further his own goal of killing everyone else, he is the truly 'evil' presence in the game.
I don't think anyone can really call 343 Guilty Spark "evil". He's an AI abiding by the protocols that he was programmed to obey, and he's a pretty tragic figure too since he lost all sense of purpose, had a chance to regain it and had it taken away again.