Thank you, heypresto, for your quick reply.
I'd also assumed that X was the perpetrator, but learned later from my correspondent that X was the victim.
It will be interesting to see others' assumptions.
I doubt I would use the term "X's patricide" at all because of the potential for confusion. There aren't a lot of examples to go on, but I think if I heard "X's homicide" or "X's regicide", I would expect X to be the person who was killed (of course, with regicide it is usually obvious whether it is the killer or the murdered man being talked about). The only time this form is regularly used is "X's suicide" where there is no confusion.