[demsux]

She arrived at the VD's with Andy in the car.

Sometimes the perps like to watch "the action"...just like an arsonist.

His prints, semen, dna, etc. are already in the house...as a friend AND he wasn't there the night before.

[juzcuz]

He could innocently return to the scene of the crime with Denise without arousing any suspicion. You're right, his prints and DNA would already be in the house, because he's been partying with them for years.

Also, the best revenge is a subtle one. If you really want to get to someone the last thing you want to do is put that person on guard against you. If he was smart, he be as amiable as possible.