Harry is a demon. Demons have desires: Excitement, pleasure, thrills , stuff like that . There are two ways for demon to have the pleasures. First is torturing, killing, destroying, war, bloodshed, slaughters. Second is sexual pleasure. Evesria was not interested in creating war nor rebellion , let alone killing. Harry didn't tell her about the second option to satisfy the demon desires ( because he sees Evesria as 'crumbs' in another world, a child. Because he is 2000 years old and Evesria is 18. Our good demon holding back). So the demon's desires are getting hard to control. Harry didn't get to kill or torture anything so he went 'insane' and unconsciously bit Evesrias neck (and lickedddd ittt uhuheheueh) in sexual way.