The answer to this question is subject to the feelings and thoughts of individual audience members or readers. You are certainly welcome to argue that Mercutio should share some of the blame for the eventual deaths of both Romeo and Juliet. A person could argue that had Mercutio not fought Tybalt, he wouldn't have died. If Mercutio didn't die, then Romeo wouldn't kill Tybalt for revenge. If Romeo didn't kill Tybalt, then Romeo wouldn't be banished and cause events to spiral out of control. Personally, I don't agree with that reasoning. If Mercutio is partly to blame, then I also think it is appropriate to blame the weather. If the weather wasn't hot, Mercutio wouldn't have been stirring for a fight.
The day is hot, the Capulets abroad.
And if we meet, we shall not scape a brawl,
For now, these hot days, is the mad blood stirring.
If Mercutio gets some of the blame, then I think Benvolio deserves as much of the blame as well. He convinces Romeo to go to the banquet. If that didn't happen, then...
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