He leads Romeo to the party. He convinced Romeo to look at girls. He also is not affiliated to either house. This meant when he and Romeo went to the party he was welcomed along with his guests. Had he been affiliated with either house closer (Being related of course) They would have either been killed or turned away. Romeo would not have met Juliet in this alternate history and they would have gone their happy little ways to not die in a crypt. In the event Romeo never met Juliet he would have fallen for another girl, possibly marrying her the politically correct way. In this play few to no people would die, Because it would no longer be about the Capulets and Montagues feud. For this I believe Mercurios actions caused Their deaths. He did not know what he had started, But it ended with his death and several of his friends. However, You could argue he wants them to die, With him Wishing “a Plague on both your houses.”Act III, Scene I Line