Who is the patron saint of murder victims?

I doubt there is one.

I say this because a Patron Saint is someone who is regarded as a heavenly intercessor on behalf of a group. They gain their status of Patron because they were either of that group when alive, or had some connection with the activities/role of the group.

They are prayed to by members of the group of which they are Patron. That is the important part, and - being dead - a murder victim cannot pray to anyone.