You want to protect them by leaving them to their own devices. Here. When there are beings that are just like Diva. Those that bring just as much risk as you do. Even someone like myself who might just lose themselves completely someday. Don't you think I fight with myself about this sort of thing every single day?
[This is exactly why he wanted to get Yuuki into this sort of thing. He wouldn't be here fighting if it wasn't for her. He would have taken himself out long ago.]
You're in a world where anyone can arrive at any time. There's no promise that Diva won't return after today. There's no promise that you won't either.
You killed your sister back home, only to arrive here and have to deal with her again. Ichiru spent time here too. The point is, there's no promise that a week from now that either of you won't be pulled back in here.