I think you'd call someone a monster too if he attacked and wiped out your kingdom and people, presumably without provocation (that we've seen anyway). Even for someone like Jillian who resented her father and her people's way of life, she still had the life she wanted and it was taken from her (apparently) unjustly.

As for the other factions, well I'm not sure beyond the royals who must see his regicide as a monstrous act.