why did Billy spare Karen Wheeler?
He didn't seem to have any loyalty to his coworker Heather.
He almost did kill her. We saw him visualize doing it. Billy was still fighting for control at that point, which is why he told her to stay away from him and walked away.
sharemaybe she reminded him of his mom
shareThat could certainly be part of it.
Billy's mom: https://i.imgur.com/cmtCswq.jpg
Karen Wheeler: https://i.imgur.com/dhFQD6L.jpg
Yep, Billy's got issues!