Some people may think that the messenger has the most fault in this situation. I can understand why because he was late to give the letter that explained Juliet's plan to Romeo. However, I think otherwise.
Friar Lawrence is the first person out of the two people
that I chose to blame for three reasons. One, Friar Lawrence gave the potion to Juliet. He gave the potion to Juliet so she could look dead so people would think she had died just so she wouldn't have to marry Paris; instead, the word got around that Juliet was dead and eventually the word of her death came to where Romeo was. My second reason is Friar Lawrence married Romeo and Juliet. Since Friar Lawrence married the young couple they now think that they're suppose to spend every minute of their life together; so if one of them dies the other will too.
Although Friar Lawrence had fault in this situation, he's not the only one. Tybalt also has fault in this situation. Tybalt's fault starts back when he challenged Romeo. Instead of fighting Romeo he ended up fighting Mercutio. When Tybalt and Mercutio were fighting, Tybalt ended up killing Mercutio even though he didn't mean to. After Tybalt killed Mercutio, Romeo got mad and felt that he needed to get revenge on Tybalt. Romeo ended up fighting Tybalt and killing him. When Romeo killed Tybalt he thought he was going to get executed; instead he got banned from Verona since Tybalt killed Mercutio. If it wasn't for Tybalt challenging Romeo, Romeo nor Juliet would've committed suicide.
Friar Lawrence and Tybalt had the most faults in the situation. I think this because it all started back from marriage and challenging. If it wasn't for them two Romeo and Juliet would've lived happily ever after.