I have always thought it was random victims not random house, I think he knew of the house, knew it was a house that was well known within the university so would be a big media storm, I also don't think he intended to kill more than one, just that night two were in one bed upstairs, and Xana must have come upon him as he was coming down stairs so they saw each other and he chased after her which is why he killed her in doorway where he caught her, I think Ethan woke up or at least stirred so he heard him this he killed him,
If he and Xana did not cross paths IMO she and Ethan would be alive, he killed them out of necessity as they could have called police, I don't think he even knew Dylan was there and saw him, it's a ridiculous theory posited that he may have been too exhausted to kill her, he has just stabbed 4 people to death his hands would have been dripping in there blood and he just couldn't bring himself to kill a person who could have been calling 911 before he even got out of the house (He was probably wearing gloves so I didn't mean blood was literally on his hands)