Basically when Kat tells Ronald that Sebastian hit her, there's a deleted scene in which he DOES. So she wasn't lying.

It was cut so show Kathryn as more of the villain and not Sebastian. So his death would appear.... like he was just "innocent" in this? I don't know. He wasn't a good guy. She wasn't good, either. Neither of them were good, likeable people. He dead a terrible person. Trying to make Kat "worse" is just stupid.