Even comic book Lex has his memories of Smallville...he just has chosen to completely ignore them. Only Lex's childhood memories of being in Smallville were wiped, not all of them. As to what motivates him...Lex is still the psychopathic/megalomaniac he has always been, just without the Smallville memories. It kind of was like an anti-mirror for Martha's comment about Clark not forgetting his time in Smallville. For him those are what defined him. Lex doesn't need his Smallville memories to define him. I think that was another point of that scene, but of course I'm delusional so what I know?