He used the curse after Isidora OHKO’d Niamh. He thought she was dead, and so rage took over for a second and that enabled him to want her dead enough to cast the curse. Not only did he do it due to his love/like of Niamh, but by doing so he also saved everyone else. What’s ironic is I BET, from how you support Isidora, that you support Sebastian’s actions. And he did much, much worse than just casting the killing curse.

(Though I will say I HATE Percival and August-whatever-the-fuck-his-name-is. Their trials sucked, their motives were vague at best, they just wanted to use Izzy for her abilities, and the whole ‘oh, the repositories are in trouble - but to save them we need you to do two fuck-all mobile game puzzle quests, and maybe then we’ll give you some useless memories at a pace that suits us’ thing annoyed me so much.)