Sentinel and Optimus have always had a bit of a rivalry. The incident involving Elita-1 pushed it to its breaking points, and Optimus Magnus' recent promotion brought Sentinel over his limits. Having gone rogue, the former Elite Guardsmen turned away from the Autobots, but his honor preventing him from siding with the Decepticons. A dangerous wild card, Optimus Magnus knows his former ally must be stopped, no matter the cost. Note: I do not claim ownership to the lineart, I simply filled in the colors.