I still think Baam learned the truth about the tower, and he was most likely imprisoned by the Zahard family in that cave. I also don't see Baam as necessarily evil or that his attitude has changed. Previously he would have dropped anyone if they hurt Rachel but now his goal and motivation has changed. Normal people that climb the tower are for Zahard, therefore, everyone that climbs the tower is against him and his goal. Also he did not kill the five or six people that have taking the test with him. The chapter that he reflects on Rachel becoming his friend and he thinks about becoming friends with new people... gives me the impression that he still is friendly. Baam is just much more confident in himself, and maybe in this next chapter of playing a game with the adim he will learn to be a little more reserved and work as a team.
I don't really buy into that.
I think it's pluasable that the Zahard family could be in some way responsible for Baam being shut in a cave. But I don't think that would even come close to explaining Baam's new personality and motives.
Baam, as he was presented in part 1, was never really angry about his existence in that cave. It may have been terrible but that was the only world he knew. If nothing else being locked into that cave allowed him to meet Lahal and I doubt he regrets that even with all that's happened. So to derive a personal vengeance motive from that would seem pretty weak to me. Even if they killed his family or something, what's the point of a avenging a family you never even knew? That just doesn't carry much narrative weight in my opinion.
Something that I think would make more sense, given his personality and mindset, would be if he decided to blame what happened with Lahel on the tower itself. Like thinking the tower somehow corrupted her and as the Zahards kind of make the rules perhaps he thinks destroying them would be the first step to changing the nature of the tower.
Unfortunately, there are problems with that too. Namely that Baam is personally friends with several Zahards and the idea of the Baam from part 1 turning on his friends is absurd. I mean, if he was just after the king; then that would make sense, but he isn't. He explicitly said that his purpose in life is killing Zahards
and their kin. And that just makes no sense to me given what I know about Baam.
And none of this would explain his ruthless attitude toward other climbers. They are NOT necessarily friends of Zahard. Baam should know this because he knows Anak a person who is driven by vengeance for her mother against the Zahards.