Not getting laid sucks, especially when it's not by choice.
So it not being able to have a girlfriend.
You can feel really left out, but I don't get how that can relate to murder.
Ted Bundy could pick up women easily and preferred to murder them.
The Green River Killer was married a few times and had several girlfriends so did BTK. Ted Bundy was also very handsome, not just average or good looking.
Calling people ugly is stupid too. Plenty of people call Daniel Craig ugly, and said he was too ugly to play James Bond, yet he's considered one of the sexiest James Bonds in the series.
It's not easy for most guys to get women. I have a friend who looks just like Johnny Depp, people tell him that all the time and are always like "You must have an easy time getting girls." Yet he doesn't, he says it's even more frustrating having people tell you how good looking you are all the time and still not being able to have luck with women.
But he is also one of the nicest, and generous people I know, his sexual frustration doesn't lead him to violence.