Who or what is the main factor that caused Romeo and Juliet to die? eg. the hatred between the two families or a person's actions or intentions?
The answer to your question can be found in the Prologue.
"Chor. Two households, both alike in dignity,
In fair Verona, where we lay our scene,
From ancient grudge break to new mutiny,
Where civil blood makes civil hands unclean.
From forth the fatal loins of these two foes
A pair of star-cross'd lovers take their life;
Whose misadventure piteous overthrows
Do with their death bury their parents' strife.
The fearful passage of their death-mark'd love,
And the continuance of their parents' rage,
Which, but their children's end, nought could
Is now the two hours' traffick of our stage;
The which if you with patient ears attend,
What here shall miss, our toil shall strive to
The Prologue tells us that Romeo and Juliet will fall in love and then die. Their destiny is fated, or dictated by the long standing feud between their two households. Technically, the feud is responsible for the structure that results in their deaths, but their individual actions also contribute to their deaths.
But consider, if the feud, which I believe is the main factor, did not exist, then Romeo and Juliet would not have had to hide their love or their marriage.
Literally, of course, Romeo drinks the poisoned tonic thinking Juliet is truly dead, and Juliet stabs herself when she awakes upon seeing Romeo's corpse. Prior to their death, there are missed messages. The timing was not right for the 'star-crossed' lovers, so Romeo is not aware of Juliet's fake death. However, more importantly, their death is the result of the feuding families who could never conceive of a Capulet and a Montague marrying. Their love for each other illustrates that a family name has little importance to love: " A rose by any other name would smell just as sweet." Their death is the direct cause of the feuding and the hatred (enmity) between the two households. However, they are also responsible as they met and fell in love by stepping outside the boundaries of what was permitted by their families.