He wasn't actually trying to kill us though, at least story-wise. His dialogue during his fight has him say to Sebastian, "I am protecting you, you foolish boy!" Then he says to our character, "how dare you? You're no friend to my nephew!" It's also probably safe to say that the whole part about the relic requiring a dark sacrifice means sacrificing a life.
The only explanation we get for why Solomon hates the Dark Arts so much is that he was a former Auror and he abruptly quit after he used an Unforgivable Curse in a fight against dark wizards, which horrified him. I wouldn't really call it a false moral dilemma, but I can agree that the game heavily leans towards Sebastian's side up until he used the killing curse. Especially since the asswipe doesn't even thank us for saving Anne from being skewered from a goblin and exiles Sebastian. Excuse me bitch? Fuck off with your "your father would be ashamed" and "we don't use Dark Arts even against our enemies" bullshit. Unforgivable Curse or not, be glad your niece is still in one piece, you ungrateful shit.