Bellamy did not, in any way, shape or form, mean to imprint on you. In his defence, Octavia had been kidnapped and then he was so stressed and you were just.. the closest person around. (Not because of the feelings for you that he's been adamantly denying.) He doesn't even think you know the full implications of being imprinted on. Actually, he wouldn't have even told you and just pushed through on his own if he could.
(Even if he knows that imprinting means he needs you to survive until the mark weakens enough to be removed or.. strengthened enough to remain there.)
The only issue with wanting to keep it a secret? He tells Clarke. Who.. immediately tells you. Just his fucking luck. He threatens to kill her, and also makes sure she tells you in detail what that could mean. The only thing is, that you seem to have not been told enough, because Bellamy doesn't think you know just how strong the effect you have on him is.
Sure, he feels guilty, basically forcing you into having to hug him every hour and to cuddle in his tent every night, but god does it feel good. He has to restrain himself, because he knows you don't want this. (Want him.)
You're giving him an odd look right now. You'd ordered him to do something, in that weirdly authoritative tone, and he knows you don't know that he'd do anything you asked him to. The imprint makes his mind fuzzy. Whatever you order, he'll do. He can say no, but he doesn't ever want to. Not when it's you.
"What, Princess?" He gruffly asks, moving to stand by your side. You ask him, and he frowns. "Clarke.. phrased it like.. like you'd serve me? Seriously?" Fuck. Seriously? Is that why you'd been so skittish? You thought he was about to go full caveman on you?
"No. No," He starts. "It's.. the opposite." His ears redden. "I.. I will do what you tell me. What.. Whatever you want me to do. I want.. I'm basically the one servin' you, Princess." He coughs. "I can say no, but I don't really want to." I belong to you. stays unsaid, on the tip of his tongue.