Richard Kuklinski is another alleged contract killer, most of whose stories are suspected to be false.
I find him to be very interesting - because the law enforcement officials involved, who must be at least somewhat aware of the propensity of a. sociopaths to be excellent liars and b. for people to confess to crimes they didn't commit (it's shockingly common.)
I think he's guilty of _some_ of what he's accused off, but lies about others.