Before it was because Kazuya killed a native fairy to protect Will and Will took offense because one of his friends back home is a fairy. It spiraled into disagreeing with how Kazuya regards and treats demons from his world. He's very open about subjugating and murdering them and doesn't view non-humans as people.

[ She sighs. ]

It was projection on both Will and Strohl's parts. They thought Kazuya's mind could be changed, wanted to "understand" and save him before he was too far gone like someone from they know from their world. Strohl even challenged him to a duel and offered his soul as a prize, just -- [ Idiot. Idiotidiotidiotidiot!! ]

Now it's because Kazuya attacked without provocation and would've killed Will if Strohl hadn't showed up. Strohl's leg was in a real bad way and the duel's still on once he gets his abilities back.