Kalliyan and Revalor had a back story, a positive past relationship as they fought together against the demon in the past. The King of the Elven Kingdom and the Guardian of the Gigante Forest.
Revalor looked up at the majestic figure who he had not seen ever since he came to the Earth. He did not know where Kalliyan was, so he did not put Kalliyan into the factor in his plan, "For the sake of our past relationship, can you let me go, Kalliyan?" That was his last resort. He did not think of making his way by defeating Kalliyan. Even in his prime condition, he did not think he could kill The Kirin.
"I will not kill you and your son for the sake of our past relationship, but I can't let you go after what you have done, Revalor," The Kirin will not let the former Elven King escape under her watch. If her Master found out that he let Revalor go after killing Virion, then she might be killed by her Master. Even if she was not killed, she would receive a heavy punishment.