I was in the depths in front of Domhnall of Zena when this slime thing appeared right in front of him. I stabbed at it but also hit Domhnall and I was forced to slay him. I know he sells boss armor which really bums me out but is there any other way to get that armor? If not is there comparable great armor in the game or should I just restart? I'm kind of a sword/sorceror character. Thanks.
He sells the master key, which isn't that big of a deal by the time you can buy it anyway, as well as boss armor like you said. Honestly, boss armor is more for aesthetics than function, and you can always buy it on NG+ since all NPCs are restored.
One of the important things he sells is the Master Key(I never take it as a gift) but most of the boss armor I believe can be outperformed by other armors. I use them mostly for looks, like Artorias and Ornstein. So unless you wanted a particular armor you're fine. The master key isn't needed and he'll return in NG+
Monster Hunter Tri: Maria - HR 121 Monster Hunter 3 Ultimate: Ultima - HR 5 :( NNID: UltimaVanguard
You are clearly not ok. Has Lordran already driven you into madness? Have you begun falling into the undead craze that will lead you to attack any living being? How could you. Now a new hero has to slay you, as you are no different from the other undead.