Answer:
The Prince blames the feud between the Montagues and the Capulets for the deaths of Romeo and Juliet because he believes the deaths of both families' children are a "scourge... laid upon (their) hate" (line 292)
The first character who is to blame for the deaths of Romeo and Juliet is Romeo himself. The first reason Romeo is to blame is that he went uninvited to the Capulet party. If Romeo did not go to the party, Romeo would have never met Juliet, and Tybalt would never have known he was there
He pronounces that the tragic deaths of the two lovers are a punishment for the hatred that the Montague and Capulet families have allowed to flourish. In the matter of Romeo and Juliet's death, the Prince rules, "Some shall be pardoned and some punished" (5.3. 319).