Are Romeo's own decisions ultimately responsible for his and Juliet's deaths in William Shakespeare's play Romeo and Juliet?
There is plenty of blame to go around for the deaths of the young couple in William Shakespeare's Romeo and Juliet, and of course Romeo must take some of that blame. What is also true, however, that the adults in their lives have to takes some blame, as well.
First of all, the Friar has to take a lot of blame for the young people's deaths. He is the one who unwisely marries Romeo and Juliet though they have known each other for mere hours. His justification/motivation is that he hopes this act will end the feud, but performing this secret marriage undermines their parents' authorities and is not likely to accomplish this goal. Then, at the wedding ceremony, the Friar gives Romeo this advice:
These violent delights have violent ends
And in their triumph die, like fire and powder,
Which as they kiss consume: the sweetest honey
Is loathsome in his own deliciousness
And in the taste confounds the appetite:
Therefore love moderately; long love doth so;
Too swift arrives as tardy as too slow.
This is a mixed message to a young man, at best.
The Friar also has to take some blame for the unlikely plan he formulates to get Juliet to feign death so he can take her in secret to Romeo. Without this plan, Romeo would undoubtedly have been miserable in Mantua and Juliet would have been married to Paris, but at least they would be alive. It is a ridiculous plan with too many variables which are not in his control.
Finally, he has to take some blame for Juliet's death, as he runs like a coward when he hears voices approaching the tomb. She has a knife, and he had to know she was desperate and rash. Her death is almost more his fault than Romeo's, and some might make a case that her death is entirely his fault.
The Nurse has to take some blame for the tragic deaths, as well. While Juliet deliberately leaves her out of her final plans, it is the Nurse who is, like the Friar, complicit in the secret wedding which marks the beginning of the end for the young couple. She is a terrible adviser.
In the end, however, Romeo wants to marry Juliet and he makes it happen. Juliet takes no convincing, but Romeo convinces both the Nurse to bring Juliet to the church and the Friar to marry them. Then he kills Juliet's kinsman, Tybalt, out of anger, despite the Prince's edict and proposed punishment. Instead of seeing his banishment as the gift of life, he whines and cries like a child. He is not to blame for the delay in getting the Friar's letter (the blame for this reverts to the Friar), but what he does after that is entirely his choice. He is the one who chooses to end his life.
The same is true for Juliet. She takes her own life; however, she would not have done so if Romeo had not committed suicide first and/or if the Friar had not left her alone in such an emotional state.
Your question is specific to Romeo, and it is true that he is probably technically to blame for both his own and Juliet's death. What is not factored into this conclusion, however, is the contribution of those who should have been wiser and given him (both of them, really) better advice. The adults in their lives let Romeo and Juliet down--including their fathers who persisted in their ridiculous feud. All of them must take blame for the result, which is exactly what happens after the deaths are discovered.