I mean, he seems like a good guy in Wyatt's story but that was day 41. It's been 359 days since then. When he mentions why he shot Nate's friend, he says that it was like he was " on speed or something". Seeing as how he obviously regularly used drugs, couldn't that mean he kind of enjoyed it? And since drugs must've become sorta rare by now, maybe he's started killing people just for the thrill?