Yeah, this was a successful episode. I feel like it was depicted quite well what would happen in such a situation. The country is in chaos. Flamingo is quite a gambler. He got a totally free and huge workforce and at the same time got perfectly rid of all enemies and nuisances since they disappeared from people's memories, but the risk was naturally equally gigantic.

Isn't Law shackled with sea stone? I doubt he could do anything even if he wanted. So, yeah, I'd say it was Flamingo himself or some subordinate of his. It would be quite a stretch for a person himself to control beheading, unless it's a power like Buggy's, but who knows.