After you kill him he actually dies this time (trust) and since he is the final boss of genesis part 2 and genesis part 2 is the last dlc of ase, then you have beaten the entirety of ase.

Rockwell saw a strange flashy rock and picked it up to examine it , whilst continuing to walk, unaware of what’s in front of him and he and the rock fell in a well, a rock well, you could say, he then mutated into the gigantic mutant monster we all know him for, - The End

So this is Rockwell prime?

A few more words