That resolution does seem a bit disappointing. Burn Gorman is perfect casting, though!

He was great! He has to carry the majority of the film's ambiguity and he does it effortlessly. Shallowly, it may be the dowdiest I've seen him: their costume department very carefully chose one of the dullest windbreakers known to man.

An alternative ending did not immediately suggest itself to me, but preserving the distinction between the stalker and the serial killer felt important.