Even the most hardened killer or most horrible monster can have someone who cares about them, and who will mourn their passing.
We'll just leave that there to sink in, for when you need to come up with a challenging plot after the heroes have successfully slain the Big Bad of their current adventure.
Malakili: You... you killed him.
Luke: Thank goodness.
Malakili: Buh... buh... How would you feel had someone brutally slain your master? <sob>
Malakili: None was ever such an artiste at the slicing trade. I sought out my apprenticeship three years ago and he taught me but a sliver of what he knew.
Malakili: But what would you know of the death of a mentor about whom you care? <sob>
Luke: Oh. I... I'm sorry. I had a mentor once.
Malakili: But was he brutally cut down by a merciless Jedi?
Luke: Look, man, I don't want to upstage your unhappiness, but yes, he was.
Malakili: But was he a refined gentleman who dedicated himself to the art of slicing?
Luke: ... From a certain point of view.