Tybalt is Juliet's cousin, i.e. a Capulet. After he kills Romeo's best friend, Mercutio, in a street brawl, Romeo mortally stabs him. This causes Romeo to be banished from Verona. Tybalt majorly contributed to Romeo and then Juliet’s death. If he hadn’t killed Mercutio in the first place, Romeo wouldn’t have been banished and Juliet wouldn’t have had the potion.
Tybalt, the "Prince of Cats," tends to stalk around proudly looking for fights. There’s no explanation for his aggressive behaviour, although it seems possible that he’s so eager to fight because he wants to defend his reputation as the toughest of the Capulets. If there’s a persona of hate in Verona, it’s Tybalt.
In fact, due to Mercutio’s death from Tybalt’s hands, Romeo killed Tybalt. This resulted in the banishment of Romeo from Verona. If Romeo wouldn’t have been banished, Juliet would not have drunk a potion to fool her family that she was dead. Romeo wouldn’t have committed suicide which means that Juliet would also be alive!
Tybalt spotted Romeo at a Capulet party. Furious at this invasion of Capulet territory, he wanted to fight with Romeo right then and there just for being a Montague. Lord Capulet intervenes and Tybalt has to obey him, but he swears to punish Romeo at the earliest opportunity. As he says, "I will withdraw but this intrusion shall now seeming sweet, convert to bitterest gall" This clearly shows his unnecessary greed for blood-shed!
Tybalt looks for Romeo and finds Benvolio and Mercutio. He purposely tries to provoke Romeo into a fight. Romeo responds mildly to his abuse and refuses to fight. Mercutio challenges Tybalt to a duel instead. Romeo tries to stop them from fighting, and Tybalt wounds Mercutio while he is distracted by Romeo's interference. Romeo then challenged him to a duel to avenge Mercutio's death. Romeo killed Tybalt and was