Though I'm the last person to condone the actions of a serial killer, assigning his motivation to his allegedly small penis is a very strange train of "logic". The majority of men are in possession of average sized penises (hence the word "average) and most of them are at least satisfied with the equipment they were born with. I've never seen any studies linking the size of a man's penis with his propensity to kill in cold blood, and I doubt that any such study would stand up to the rigors of peer-reviewed publication.
As has been mentioned previously in this thread, the size and shape of his actual penis is hardly a matter of photographic record, at least based on the links provided. Claiming that he has a micropenis is quite a leap here given the evidence as we have it.