I would have considered Bellamy using Clarke to convince Madi as
super manipulative if he hadn't told Madi flat out that Clarke didn't approve of the plan.
Yes, she's a child so there's a different power dynamic at play there. But I think she really does understand the gravity of taking the Flame. So I'm hoping her telling Clarke that it was her choice will help heal that feeling of betrayal a bit.
And I also absolutely understand Clarke taking Madi and running. I mean, realistically what could she do at this point to stop Octavia? She doesn't
know how many members of Wonkru would hypothetically support Madi. And Octavia is back and clearly out for blood. Her taking Madi and running is at this point the only way to immediately secure her safety (as is made clear by Octavia trying to have them both killed.)