My experience of mercenaries is that they are not too picky about whose money they take.
Who's going to pay them to kill Putin? Most likely not the EU, nor China, but maybe maybe maybe the US. However that wouldn't go over well with their war industry as a longer war in Ukraine means more money. It would have to be some private billionaire who's hurt so much by the war that he (and maybe his friends) decide that Putin is costing them more money than it would cost to dispose of him.