Whose fault really was it that they would end up killing themselves. Well there are a number of possibilities but there are a few that really do make more sense than others. The characters that more so played a larger role in Romeo and Juliet's deaths are Friar Lawrence, the Nurse and the Capulet parents.
Friar Lawrence was by far the most at fault because he married the two kids together in the first place thinking only of good possible outcomes. When things started getting tuff he came up with a plan that asked far to much of Juliet, a crazily in love drama queen just becoming a teenager, and Romeo, a young, spoiled, over the top emotional boy with raging hormones. It would have taken a miracle for his plan to work. In the end to top it all he didn't even get to Juliet before Romeo, which topped his list of mistakes.
Now if this guy isn't guilty the people who have read this play have some serious judgment issues.
The nurse was neck deep in this entire incident because not only did she assist in Juliet marrying Romeo but she also didn't take Juliet's side when she found out about the arranged marriage to Paris.
Although I said the Capulet parents were at fault they weren't knowingly creating such problems for Romeo and Juliet. But never the less they were at fault because of the fact that they were so forceful and urgent in making Juliet marry Paris. Another reason for their involvement is having the feud with the Montagues. This feud led to a lot of hate, the innocent deaths of Tybalt and Marcutio, Romeo's banishment and the needed secrecy for Romeo and Juliet to get married.
All of the previous characters were heavily involved in the deaths of Romeo and Juliet as demonstrated in each paragraph. In each case they're all guilty Friar Lawrence because of all his huge screw-ups, the nurse because of her unwillingness to not just do what she's told and actually making good decisions, and the Capulet parents because of their inability to have a little bit more patience with other people and there daughters decisions.