It could be on the orders of his patron, rather than anything personal against Karlach/infernals.
I wasn't terribly invested in Wyll so I'm curious to see where this rewrite goes.
They have what I think is a new write up for him on the official website now, unless I am misremembering.
It is noted that his patron specifically is asking him to sacrifice devilish creatures and hunt her enemies. I imagine when accepting this deal Wyll said "Well whatever - I want to be a hero and kill evil stuff anyways" and she is going to start asking him to kill not so evil stuff (Karlach - as example).
