Answer:
In these lines Romeo is threatening to kill Paris, and his other sin is to have kissed Juliet.
Explanation:
In this scene we find Romeo in a fight against Paris. He warns him that he does not want to fight, but that if necessary he will defend himself and will have to kill him.
Romeo always sought peace with Paris but Paris decided to fight anyway, and Romeo had to kill him. This was what produced his exile.
On the other hand, the other sin that Romeo has was having kissed Juliet. Let's remember these lines:
"If I profane with my unworthiest hand
This holy shrine, the gentle sin is this:
My lips, two blushing pilgrims, ready stand
To smooth that rough touch with a tender kiss. "
That would be his other sin.
