I disagree. While he certainly didn't mind killing Luz, compared to his other deeds, rejecting redemption is basically nothing.

Back to the question, I think it's when the murder is preceded by torture/beating/rape, when the victim is a small child or when the death itself is really gruesome.

Your answer to question is not the case

I mean why Kikimora’s attempted murder of Hunter and Odalia’s attempted murder of Luz is MEH, but Belos killing Luz in the finale is not

Because int he cases of Odalia and Kikimora's had theirs happen in the beginning of their arcs, with the former to show she is more than a "mere" karen, and the latter to show she ain't loyl to the Coven.

Belos on the other hand, had "Hollow Mind" already out to establish how much of a horrid and unreasonable excuse of a human being is. They are very different scenarios

Fine then (not rudely saying this, just accepting this)