Great thoughts here, everyone!
I saw it as a bit manipulative. Not in a direct sense, because, well... he and Gaia told Madi the truth. They didn't force it on her. But they also made it seem like it was the only option to stop the war, which... no. And I think that Madi bringing up saving Clarke was an "in." Personally, I don't think they should have gone to Madi in the first place. I also think if Madi knew Bellamy was the one to poison Octavia, she never would have taken it.
I think the slapping him/leaving him to fight in the arena actually makes perfect sense.
I'm not a mom, but even if it was like... a cousin or something, if someone I trusted completely put my family member in danger... yeah, sorry. Screw that person.