Why does romeo suddenly want tybalt dead ?

Romeo wants Tybalt dead in revenge for the death of his friend. 

Romeo wants Tybalt dead as revenge for him killing Mercutio. He shows this when he says, "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" (3.1.130-134).

the reason why romeo suddenly want tybalt dead is because he killed his best friend mercutio