Tybalt's character is synonymous with violence and aggression. For example at the Capulet ball in Act 1, Scene 5, the mere sight of Romeo causes Tybalt to launch into hostile action.
"Now by the stock and honour of my kin, To strike him dead I hold it not a sin" --> his first instinct when he sees a Montague is murderous.
"Fetch me my rapier, boy" --> immediately resorts to violence
"A villain that is hither come in spite" --> by describing Romeo as a villian, it reflects how Tybalt feels compelled to attack him: he feels he's performing a righteous deed and is fulfilling his duty