Tybalt hears Romeo's voice from behind a mask at the Capulet party, and tries to fight him there adn then, but is stopped by Lord Capulet, who doesn't want any fighting at his feast.
Tybalt resolves to be revenged on Romeo another day: Romeo's "intrusion" into the Capulets' party, though it now seems sweet, will "convert to bitterest gall".
So, in Act 3, Scene 1, Tybalt comes up to Mercutio and Benvolio to find Romeo. When Romeo appears, Tybalt declares that he's found his man, and addresses Romeo directly:
Romeo, the love I bear thee can afford
No better term than this: thou art a villain
Tybalt knows nothing about Romeo's relationship with Juliet. He's only out to get him because he went to the Capulet party. Romeo, of course, backs out, because - now married to Juliet - Tybalt is his relation and he shouldn't be fighting with him. Mercutio steps in in his place... and the play twists from comedy into tragedy.