Tybalt actually sets the entire tragic sequence in motion. Through Acts 1 and 2, Romeo and Juliet’s biggest problem has been to maintain secrecy. Their families hate each other because of a generations-long feud. When Romeo and Juliet realize who they have fallen in love with, they know they will have problems, but they cannot foresee what is about to happen in Act 3.
The act opens with Romeo and his men in street. When Tybalt and his men show up, they have the following exchange of words:
Follow me close, for I will speak to them.
Gentlemen, good den: a word with one of you.
And but one word with one of us? couple it with
something; make it a word and a blow.
You shall find me apt enough to that, sir, an you
will give me occasion.
As we can see, Tybalt intentionally makes verbal contact with Mercutio. Had he not done that, the groups might have passed each other peacefully in the street. However, Mercutio is equally hot-headed, and immediately suggests a fight. The fight results in the death of Mercutio, which leads Romeo to kill Tybalt, which forces him to flee Verona. Eventually the cause and effect chain results in the deaths of Romeo and Juliet.
Romeo’s impetuousness is of a romantic bent. He is taken by Juliet to the point of being swept away. There is not violent element to it until he sees Tybalt kill Mercutio.