Manik wanted her to say the truth but the thing is he thought she trusted him enough to know he would never kill someone. He can't be that cruel. He wouldn't be so shattered if it was someone else but it's HER that matters. He didn't expect her to doubt him. Not after all this. Not after he has given himself to her, entirely. Not when she has seen the real him. It's that 'doubt' that kills him. We all know what trust means to Manik. To him, love is trust. And the fact that lady he loves didn't trust him breaks him.
And what makes all this worse is that he looked so happy sometime ago. SO happy in love with her. He was finally ready to create a new world with her. Enjoy his life with her. And he got a reality check. GOSH. That HURTS.