Tybalt and Mercutio were similar in many ways, one of which is that they were both hot heads looking for a fight. So one could read the scene as two equally matched young men whose fates would be determined by which man was faster or better skilled. Tybalt probably did not want to kill Mercutio. He really was after Romeo. And surely Romeo did not want to be responsible for pushing Mercutio to the point where Mercutio would die.
These were the circumstances surrounding the fight between Tybalt and Mercutio. The question of who was responsible, however, appears to have one element missing. What about Mercutio himself? Where does his responsiblity in this affair come into play? Are we not responsible for our own actions? No one forced Mercutio to take action. He did so of his own free will. Did he feel that he had to protect Romeo or to save his own honor? Maybe. But he and he alone made the decision to pull out his sword and go on the attack. Was he crazed at the moment? Possibly. But this does not diminish his responbility. At some point, Mercutio made a decision, and from that point, circumstances played against him and he was forced to face his death.