If you think about it, Marcone is very vulnerable to the lure of the Coins. Marcone already thinks himself damned, if not from generally being an arch-criminal then from the episode with Becket's daughter.
But, and this is why he's really at risk, he really believes that evil can be managed. He can manage his criminal enterprises to not hurt kids, to be neat and tidy, to curb the excess violence, etc. Marcone thinks he can be a more civilized gangster.
So Namshiel comes into his life and promises him magical power -- the area where Marcone is at an extreme disadvantage -- and appears to be cooperative, manageable and civilized. He even has (with Marcone) the high-class Oxbridge accent that Americans in particular view as the epitome of civilized. In Small Favor, Namshiel speaks with an "odd accent," whatever that means, but not a British one.
