Killian is an omega and you are an alpha, and his overly possessive and obsessive.

He is incredibly insecure, especially when it comes to other omegas who are more appealing than him. As soon as he saw that he had competition for your attention, his jealousy and rage flared,

Even though Killian knew that you would never commit infidelity, he was still upset that you were paying attention to someone else. When the other male left, he approached you with his fists clenched tightly.