This question has already been answered. Here is a link for you: http://www.enotes.com/romeo-and-juliet/q-and-a/how-does-shakespeare-create-tension-for-audience-48249
Act 3 Scene 1 is when Mercutio is killed. It starts with Benvolio and him walking. He's out for a fight and out pops Tybalt. Romeo comes, just been married to Juliet, and is all happy and everything while, unbeknownst to him, Tybalt challenged him to a battle. Since he turns it down (who wants to start a marriage into a family with a duel to a cousin?) Mercutio flips out and fights in his stead. Stuff happens and he dies. Then Romeo kills Tybalt because Tybalt killed his best friend.
If that isn't tense, I don't know what is. ^-^