iirc, Doflamingo told him he could do whatever he wanted but not as a part of his crew/under his flag anymore. Then he controlled Bellamy's partner to stab him in the back and we heard him scream off-screen.
With Bellamy now confirmed alive, I think the only off screen deaths left un-retconned are Tom and Gin. And bringing either of them back alive would probably upset me, unlike the he-man bellamy we got going here.