User blog comment:B214/High School DxD New Episode 3 Discussion/@comment-16075832-20130722160729/@comment-1359322-20130723031119

That's because Issei was afraid that Kiba would become a Stray Devil to take revenge on the Excalibur so he decided to take the course of action to prevent it from happening. I think it's even clearly stated in the anime, you need to pay attention to the story, didn't Issei explain it to Koneko.