Firstly, I'll say that I get why Leo is acting like a brat. The reason is that Fooly is an idiot. It doesn't matter whether Melvie's "feelings" are legitimate or not; she can't be trusted.
Yes, Leo is being jealous, but her concerns are realistic. Melvie abandoned her humanity for the sake of power; losing a fight once isn't gonna shatter that lust for power, if anything, it would make her vengeful. I don't care if "she realized the futility of it all because Leo never took her seriously in a fight," the lustful, greedy, and power hungry never think rationally.
@Tamerlane It's definitely a case of the mangaka developing "Nice Guy Syndrome," he thinks all women will wanna fuck a guy if he just shows them some common courtesy.