Who is guilty?

I would have to say that when it boils down to it, whoever is holding the gun, sword, knife or whatever and has the intent to harm is always at fault. It is very true that Romeo had terrible timing and if he hadn't jumped in the way then Tibult would have been unable to kill Mercutio, but still it was Tibult who killed Mercutio, not Romeo. I mean if we want to go down that path we could say it was just as much Mercutio's fault for getting into the fight against a better swordsman. I believe the blame has to rest on Tibult and his intentions.