"By my head here come the Capulets"(Act 3 scene 1 line 34)
Tybalt, Mercutio, and RomeoStreets of Verona
"I am hurt. A plague o' both your houses! I am sped. Is he gone, and hath nothing?"(Act 3 scene 1 lines 89-91a).
As Romeo was outraged by the fact that Tybalt killed Mercutio, he has killed Tybalt as revenge (hate)
"O, I am fortune's fool!"(Act 3 scene 1 line 135a).
Benvolio and Mercutio are just having a casual talk when they see the Capulets come and realize that something bad could happen within the next few moments. Benvolio's fear also foreshadows what is going to happen.
As peace gets the best of Romeo, Tybalt, under Romeo's arm, stabs Mercutio and starts shouting that he is hurt and that there is a curse upon both the houses which hate a each other.
Outraged by his friend getting killed, Romeo lets his sadness and anger overtake him as he kills Tybalt as revenge for what he did to make Romeo hurt moments ago. Though he killed his friends murderer, he feels that fate is responsible for this.