We're all tossing around the world "kill", but with Fate Points, that doesn't have to be the case.
PC1 attacks PC2, criticals and 'kills' PC2. PC2 burns a fate point. PC2 goes down in a welter of blood, PC1 believes PC2 to be dead. The GM may well need to step in to explain how this happens, but it's all very feasible. What happens afterwards (whether other PCs tell the aggressor to back off, or the GM creates an intervention) can be down to roleplay.
If the character in question has no fate points, maybe the GM needs to intervene sooner to create a suitable scenario to save the character's life.
RP is important to me, but if the situation arose again and my friend looked at me and said "please don't kill my character", of course I would come up with a way for it not to happen.
I do think it's okay to have in-party violence, if that's what a situation ultimately degenerates into, for the above reasons.
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