Why are Romeo and Juliet guilty of the deaths in the play?

To make this argument, you would need to show how Romeo and Juliet's actions led to their own deaths and also to those of Mercutio, Tybalt and Paris.  I think that this is not too hard to do.

Romeo kills himself and Tybalt and Paris.  He is guilty of these deaths on a couple of levels.  First, he is actually the one who does the killing.  More importantly, it is his actions that cause the situations that make the deaths happen.  This is also true of Mercutio's death.  If Romeo had not been at the Capulets' ball, Tybalt would not be so eager to fight him.  If Tybalt had not wanted to fight Romeo, both Mercutio and Tybalt would still have been alive (I don't blame Romeo for trying to stop them from fighting, though).  If Romeo had not fallen in love with and married Juliet, he would not have been in the position of having to fight Paris either (because Juliet would not be "dead").

Juliet can be blamed the deaths as well.  She did not have to continue to be in love with Romeo.  She could have broken it off once she found out who he was.  Or she could have told her parents what was going on.  Instead, she continued her relationship with Romeo while still pretending she could marry Paris.  This led to, at least, the deaths of Romeo, Paris, and Juliet herself.

So I would say that the two of them behave selfishly and this behavior leads to many deaths.