The defense theory is that he was attacked by someone in the house very soon after he arrived. Colin makes the most sense to some because of his youth and apparent aggressiveness as well as his history with the deceased, the extent of which we're not really sure.
Colin is the nephew of the Alberts and son of Jen McCabe's best friend. When it was clear John was not going to recover, his body was left out in the cold sometime after the first plow went by at about 2:30 am. Can't have our boy going to prison, and besides we can control the investigation. Or the lack thereof.
I personally suspect Caitlin Albert's boyfriend didn't return to the house because she's "high maintenance" but instead to collect Chloe the biter and take her back to their house in Easton. Neither Tristan nor Caitlin explained why she suddenly felt the need to return all the way to Easton that late in a snowstorm when he was supposed to be working soon. She was in her childhood home and her bedroom was still there. They used Tristan's SUV to block anyone's view of that particular section of the lawn and brought the victim's body out from the back. This is the SUV the snow plow driver saw in his path at around 3 - 3:30 am.
Jen McCabe, control freak that she is, played quarterback. As her husband said, "Just tell him the guy never came in the house".