Death spells? That certainly escalated quickly.
And on a fellow servant of Altair, too... if Trish wasn't in trouble before, she certainly is now. Vampires aside, I'm not convinced this would count as self-defence.
If she does hit him (e.g. doesn't miss) I suspect she's going to get a very, very unpleasant interview with Altair when her time finally comes.
You know it's funny, but in the military at least it's self-defense or defense of others that we're trained are the justifiable causes for lethal force. Obviously this is more of a morality thing than a legality thing but it still seems like "I wanted to stop them murdering my friends" seems like it should be a valid reason.