We spend the whole game wanting to kill him and when we can finally do it that's cathartic!
Or if you decide to side with him (which I haven't, but other people have), you can actually fulfill your part of the bargain and watch a cutscene at the end as he goes off to do his thing (though I hear it's a bit bugged at the moment). Siding with the Chosen who offers an alliance results in basically nothing.
