Who done it?

I believe that Tybalt is most at fault in Romeo and Juliet

Tybalt vs. Benvolio

Almost as soon as Tybalt arrives he is already asking for a fight by saying, Shakespeare writes, "Turn thee, Benvolio; look upon thy death." (1,1,67) When he says this line, Benvolio is still trying to keep the peace and not fight. It is not until Tybalt's next line that insults Benvolio and his family that they start to fight. The only reason they stopped is because an officer stepped in to break up the fight and then the prince arrived and told them, Shakespeare writes, "Your lives shall pay the forfeit of the peace" (1,1, 98) that makes them stop. By saying this the prince basically told them, stop now or if you fight again it will be a death for both of you.
Romeo and Juliet(1936) - Benvolio vs. Tybalt

Tybalt Vs. Mercutio

I think that Tybalt is most at fault for the problems because he was the one who won't stop fighting and has the anger issues. Shakespeare writes, "That thou hast done me; therefore turn and draw" (3,1,68) Here Tybalt is telling Mercutio to draw his sword to fight him. When the two start to fight, Romeo, tries to stop the fight and yells at them to stop and have peace. While Romeo is holding Mercutio back Tybalt goes under his arm and stabs Mercutio and ends up killing him.
ACT 3 SCENE 1: RJ68-Mercutio and Tybalt Fight.avi

Tybalt vs. Romeo

After Tybalt kills Mercutio, Romeo is very upset and is looking for revenge against Tybalt. Shakespeare writes, "Either thou or I, or both, must go with him." (3,1, 131) Romeo is wanting to avenge his fallen friend by killing/ hurting Tybalt. Even tho Tybalt didn't start this fight (for one) he still caused it by killing Mercutio. If Tybalt only held his tongue and let things go he never would have killed Mercutio or been killed. At the same time he never said no to fighting Romeo, he basically said, well whats another death.
Swarovski Entertainment's Romeo & Juliet: Romeo vs Tybalt

What others would say

Others would say that Tybalt's not at fault, but more Romeo. They could say Romeo is more at fault because he is the one that got Mercutio killed by holding him back. That Romeo is the one that started the fight that killed Tybalt and another that killed Paris. It could all be correct but ever would have happened if Tybalt could have just held is tongue and let it fly past. But instead he reacted causing Romeo to want revenge and that Romeo wanting to die to be with his fair love, thus for killing Paris.