It's act 1, scene 5 of Romeo and Juliet, and Tybalt, as is often the case, is acting like a very angry young man. Tybalt is absolutely furious that Romeo, a member of the rival Montague clan, has gatecrashed a masquerade ball.

As a proud Capulet, Tybalt feels insulted by Romeo's very presence, so much so that he claims that it would not be a sin for him to kill Romeo. On the contrary, he would be doing honor to his family:

Now, by the stock and honor of my kin,

To strike him dead I hold it not a sin. (I, v, 57–58)

Notice here how Tybalt's words rhyme. This is entirely consistent with how Shakespeare writes for noble characters. Their dialog is written in verse, not necessarily rhymed verse as in the above example, but verse all the same. This is in contrast to common folk, whose lines are written in prose.

Though Tybalt may not have a particularly noble character, given that he's a violent hothead and serial troublemaker, he's still an aristocrat, a member of a noble family, the Capulets, so he is given the privilege of speaking in rhyme.

Such jingling rhymes are not normally used to render blood-curdling threats of murder such as the one made by Tybalt against Romeo; but this only makes his threatening words sound all the more chilling.