Why did he do it? The first thought is Necrophilia. The second thought is that he wanted them drugged to the stage that what he see's as only an object becomes unresponsive and with this being done he can do whatever he wants to without them fighting back. I think that in time Stephen Port would have become a sexual sadist. Any victim of his would become boring after a while and like the sadist the victim dies after the fun is over. I want to make a memorial here on line for all these innocent young men. This Stephen Port actually had his victim slumped against a cemetery wall, this so called 'man' is trying to make a point. Trying to introduce himself into the investigation, leaving the victim there close to his flat, makes his discovery of the body plausible.
Evidently his sickness is always going to prevail. To him that sexuality he struggles with makes him hate himself however he has let this simmer for so long he can only see it in other young men for who he feels he must kill of his own sexuality. These killers get to the stage it no longer matters if a man is homosexual or not. They present a living breathing male of who he has these feelings towards that is when they must pay the price in his mind, and they did pay they had their lifes snuffed out of them under the pretense that they were there for other reasons.
I feel Stephen Port was probably the victim of abuse at some point and that's when victim turns victimiser. He deffinately, at this point did not want to be caught. He was too busy enjoying himself.