Responses
Spoiler...
He kissed her because Roxana was badly injured internally.. He knew that cause Roxana can't stop spitting out blood and as what chapter 21 has shown to detect any impurities, and he knows Roxana's body is way too poisonous and her organs are badly injured because of her consuming way too much poison...
And major spoiler: The FL, Slyvia, Cassis younger sister has this kind of power herself.. Healing/getting impurities through kissing.. But Cassis' ability exceeds the FL.. Cause he is able to heal Roxana and getting impurities faster and better..
His sealed power that his father said to never release was just released, and it was so powerful that it killed all the butterflies and he transferred his divine power onto her body.
hope this helped in some way