How is the ending of Romeo and Juliet the result of human actions rather than fate?
A good question.
Actually, for all that the chorus calls Romeo and Juliet a "pair of star-cross'd lovers" in the prologue, just about all of the action in this play is due to human choice and action, including the ending.
True, neither Romeo nor Juliet chose which families to be born into. That aspect of the play (and their situation) could be considered fate. However, once they start acting, they are choosing their consequences, including the ending of the play. Take, for example, Romeo's actions in Act I, Scene 1. When Balthasar tells him Juliet is dead, he decides to kill himself. That's not fate. It might be passion, but it isn't fate. It also means he chooses not to do other things. Romeo does not check to see if Juliet is dead, doesn't decide to live in her memory, etc.
Likewise, when he finds the apothecary to sell him the poison, the man chooses to break the law and sell him that poison. That's not fate. That's choice. When Romeo and Paris fight (later in Act V), it isn't fated that they fight. They might have talked things out. When they do fight, it isn't fate that Romeo's the one to win, and so on.
At each point, the characters choose their ends. This includes Juliet stabbing herself.