If I kill Madanach and escape alone, and Thonar Silver-Blood will be in my debt, can I still kill him after that? I think both the Forsworn and Silver-Bloods need to be put down, I want to trim the SBs as thin as possible.

And also, I know that as a PC user I can just use console commands, but I prefer to play by the game's rules. Using console doesn't feel the same as doing it the genuine way.

I always kill Madanach and then when I exit the escape route Thonar rewards me.
He's an ass so when he turns his back to me I crouch and slip a powerful poison in his pockets, killing him.