Just a short answer to keep it as spoiler free as possible: Thorfinn does get back into fighting. It's way better after that point. The manga is about how lusting after violence is pathetic and pointless, so I'd give it a go. It's got a lot more interesting stuff to say since the slave arc.

Does Thorfinn become cool again?

As to your question, no. Thorfinn does change and get a bit better (e.g. he doesn't stand their getting hit like that again), but he's no longer raw anymore.

Does Hild kill Thorfinn?

Her master is called the Old Bear by the blacksmith in the village Hild visits (and presumably by the others in the village) and is killed by a bear rather prophetically. She also sees the bear that kills her master as Thorfinn, the man who ruined her life. ... Hild is hunted by Thorfinn then becomes a hunter of bears.