Where did Tybalt kill Mercutio?
In Act 3, Scene 1 in the street in public
Tybalt killed Mercutio because Tybalt was messing with Romeo and Mercutio stepped in to try and help Romeo. When they started to fight Romeo tried to stop Mercutio and Tybalt stabbed Mercutio under Romeo's arm. He was going for Romeo but missed; consequences were that he stabbed Mercutio who eventually died, cursing both their houses before he died