Well, one idea to consider is that if Romeo had not killed Tybalt, he would not have been sent into exile, and thus neither of them would have died. You could also consider the idea that ROmeo acted rashly by buying poison to kill himself after hearing of Juliet's death. If he had waited a few moments, he would have realized that she was just asleep.
I believe the main reason that Romeo is responsible for his death as well as Juliet's is purely and simply his rashness and impatience in nearly every event of the drama. He rushes into his relationship with Juliet, which, by the way, I believe, he sort of urges her into, then the marriage. He then kills Tybalt in the heat of the moment and gets himself banished. Now the ultimate way in which he causes both of their deaths is in his failure to wait or take time to think before taking his own life when he finds Juliet in the tomb. His immediate reaction is to drink the poison; if he had waited a mere matter of minutes, she would have awakened and they could have gone away together. However, Romeo, always the impetuous hothead that he is, refuses to wait for anything; he can't take anything that remotely resembles pain and chooses death instead. This of course leads Juliet to take the same course of action. Hope this helps.
I think that Romeo and Juliet should've known the consequences for a sudden marriage. especially during a feud of a lifetime! for Romeo, whose heart is impatient does not think much of Juliet when he almost took his life for banishment or even when Juliet suggests marriage not fully knowing Romeo! if her parents were to rage in anger because she would not marry Paris, how would they react if they knew she married her fathers households enemy?