When Benvolio told Romeo to "Go thither [to the Capulets' feast] and . . . compare [Rosaline's] face with some of the others that I will show . . . " (Act I, Scene 2, ln 88-89), he was unknowingly setting the stage for the suicides of Romeo and Juliet. Romeo would at first not go; he was depressed because Rosaline didn't love him. On the insistence of Benvolio, Romeo reluctantly agreed to go. On the way to the feast Romeo still did not believe he could find a girl more beautiful than Rosaline. When he saw Juliet, however, his attitude changed. This was the first action of all that led to the deaths of the lovers Romeo and Juliet.
When Romeo and Juliet decided to get married, they too were bringing their suicides closer to being reality. They each fell in love with the looks of the other; it was love at first sight. However, they both knew that neither of their families would accept that they loved someone from the other family. So they decided to marry secretly, not realizing that their parents might have plans to have them married. This unforseen dilemma puts the two in as much blame as anyone for their own deaths.
No less at fault for the deaths of the lovers than themselves or Benvolio was Friar Lawrence. When he first appeared in the play, he was talking of how good intentions can sometimes have bad results (in Act II, Scene 3). At the time that Romeo asked him to marry he and Juliet, he agreed to, thinking that he could marry the two then and announce it to the families later. He had hoped that this would end the feud between the families. But when Romeo was exiled, this was no longer an option. The Capulets would have been less than pleased to learn that their daughter was married to a killer. Thus, the Friar's good intentions led to bad results -- the deaths of Romeo and Juliet.
The tragic deaths of Romeo and of Juliet were the consequences of the actions and decisions of several characters. Benvolio had a part in it by telling Romeo to go to the Capulets' party. Romeo and Juliet themselves did not foresee the difficulties they would encounter if they would marry secretly. The Friar too did not foresee the difficulties when he married the two. Thus it was free-will of the characters, not fate, that was to blame for the deaths of the lovers Romeo and Juliet.