Aatrox makes a form of art only he loves, which is watching people go dark and hellbent when they are losing. What are Raast motives? Does he simply want to just "generic destroy anything near" or there is something he want's to accomplish?
Nah he just wants to kill everything. He also enjoys destroying things but killing is his main hobby.
I think its just darkin's nature ro destroy everything. Some just uave different ways of doing it. While Aatrox plans where he kills and who it affects , Rhaast just goes ballas to the walls and murders people everywhere.
He's trying to impress 
to get that ship sailing
Unfortunately they aren't interested in what he's got to offer. Too much edge lord. Too much edge.
Revenge probably given how he talks about controlling and unleashing the shadows when he takes command?
But beyond that? He probably just wants to keep on murderin.
