Grimlock fights not because he believes in the Autobots ideal of peace and freedom, but because he knows what Megatron really is. He was there at the beginning of the Decepticon movement, mudguard deep in the spilled oil on the arena floor. He knows what it is to feel the electron-surge of total victory over his foes. His coolant too has sung as he pressed his blade against the torso of a helpless opponent.
But to Grimlock, no weakness is worthy of respect, least of all that weakness that disguises itself as strength. Where is the satisfaction of crushing those who cannot defend themselves, or slaughtering a building full of unarmed innocents? How does madness like that prove one’s strength? Better to die fighting against an enemy so powerful that defeat is almost certain. Better still to turn that enemy back and drive him broken before you. To someday see Megatron so broken – that is why Grimlock fights.