Whose fault is it that Mercutio dies?

The only reasonable answer to the question lies within. Whose fault was it? The fact of the matter is, that it is YOUR fault. You, as the reader, are the one who makes fiction come to life. Clearly, in 1303, no Romeo and Juliet existed. Thus, as a work of fiction, it does not become reality until you, the reader, make it so. If Romeo and Juliet had lied abandoned on the shelf, no one would have died. When you read that Mercutio was stabbed under Romeo's arm, essentially you gave license to Tybalt in your very act of reading. Thus, Mercutio's blood, spilt right in front of your eyes, would not have been poured forth if you had not unleashed it through your erudition. Would not Mercutio have stayed alive if you had stopped reading in Act II scene vi? Thus, it is completely your fault that he dies, as you so selfishly continued reading. You unlocked the reality of the story as you kept perusing this blood-soaked play, and I can only warn you: if you dare to keep reading, more people will surely die.