No. He was trying to kill you. Imo any amount of force and any target is justified in that kinda situation. Even if you managed to slice off his balls it’d be justified imo. Granted, if he disengages and you feel safe, then you should stop; I’m just saying that, in the moment, I’m not sure I would have felt safe enough to let him disengage.
Edit: the throat is far more vulnerable than the belly. Smaller target, but significantly easier to damage something in a fatal way.
Diamond is actually a very efficient heat conductor.