When tybalt is ready to seize romeo and throw him out of the party, what does capulet say to tybalt? why? what is tybalt's reaction to this?

This, by his voice, should be a Montague. Fetch me my rapier, boy. What, dares the slave come hither, cover'd with an antic face, To fleer and scorn at our solemnity? Now, by the stock and honour of my kin, To strike him dead I hold it not a sin.

Because Tybalt is a capulet and Romeo is a Montague and they are in a feud of families.

He states why he should kill Romeo