Why did Romeo kill Tybalt in act 3 of Romeo and Juliet ? Is it because of Romeo's character? How does Tybalt's death change the action of the play?

Whenandduel,tries to
break it up. He steps between the two fighters, and "Tybalt stabs
Mercutio" under Romeo's arm (according to stage direction). Whenhelps Mercutio
to leave the stage, Romeo says of him;

This gentleman, 's
near ally,
My very friend, hath got this mortal hurt
In my behalf. My
reputation stained
With Tybalt's slanderTybalt that an hour
Hath been my
cousin! O sweet ,
Thy beauty hath made me effeminate
And in my temper softened
valor's steel. (3.1.114€“120)

Romeo is aware that Tybalt
was really coming after him, and Mercutio received his mortal injury while fighting to defend
Romeo's honor. Mercutio felt that Tybalt was speaking disrespectfully to and about Romeo, and so
he stepped in to fight Tybalt when Romeo continued to refuse. Now, Romeo seems to feel
responsible for Mercutio's death, and this sense of guilt and responsibility likely helps to
compel him to fight back.

When Romeo sees Tybalt again, he says of him,
"Alive in triumph, and Mercutio slain!" (3.1.127). He cannot allow Tybalt to remain
living while his friend Mercutio is dead. He says to his enemy:

Mercutio's soul
Is but a little way above our heads,
Staying for
thine to keep him company.
Either thou or I, or both, must go with him.
(3.1.131€“134)

Because Romeo and Tybalt are responsible
for the death of Mercutio, Romeo says that one of their souls must accompany his to the next
life.