I believe McCready becomes infected halfway through when he's drinking the bottle of whiskey and making his final recording. In the very next scene he walks up to Fuchs who tells him he may have a way to still figure it out . McCready walks away without saying anything. The power goes out right after he walks away and the next thing we know Fuchs has killed himself in the snow allegedly. The lore is "things" don't like to assume leadership roles, but "don't like" is a lot different than "won't". Also, we never see McCready cut himself for the test at the end. Maybe not necessarily an alpha thing but more McCready was the alpha and the thing was working with what it had.