What is Romeo's motivation for killing Tybalt? What are the consequences of this action in Romeo and Juliet?
Romeo's only motivation for killing Tybalt is the fact that Tybalt has killed Mercutio. Mercutio was Romeo's good friend and he is enraged and his death. I, personally, think Romeo feel guilty too because he kind of caused Tybalt to be able to kill Mercutio.
The immediate consequence of this action is that Romeo has to flee from the city or be killed. The less immediate consequences are more important. Because he has to flee, he and Juliet both end up dead.
So looking at it like that, it sure seems like a mistake. He should have let the Prince execute or banish Tybalt.
Concerning Romeo and Juliet, I won't write the paragraph for you--that's your job. I'll help you with the information you need.
Tybalt kills Mercutio, though it is in part, accidental. Tybalt and Mercutio are playing around, but it is a dangerous game they're playing. They're sword fighting, with "live" swords, swords that aren't blunted or tipped. There's anymosity between the two, but they're not really trying to kill each other.
Romeo, in an attempt to make peace and get them to stop fighting, gets in between them and inadvertantly causes a thrust by Tybalt to be missed by Mercutio, and the thrust mortally wounds Mercutio.
Romeo avenges Mercutio and kills Tybalt. The consequence is the banishment of Romeo, which of course leads, eventually, to the tragedy at the conclusion of the play.
I'll leave it to you whether or not the killing of Tybalt is justified.
Romeo's motivation in killing Tybalt is to defend the honor of Mercutio. Ironically, it was Romeo's jumping in between the swordplay of Tybalt and Mercutio that indirectly led to Mercutio's death. Had Romeo not been in the way, Mercutio would have likely been able to see and avoid Tybalt's blade.
Romeo, blind with rage, screams at Tybalt:
Now, Tybalt, take the villain back again,
That late thou gavest me; for 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.
Here, Romeo is hellbent on ensuring that Mercutio's soul does not travel alone. However, Romeo never pauses to think of the repurcussions of killing Tybalt, who is Juliet's cousin, until it is too late when he excalims:
This is the play's climax when Romeo realizes that he has just murdered the cousin of Juliet, whom he has just married.