Does Machiavellianism concept attempt to combine different personalty types?
"If they are mistrustful, why are they better in trust games?"

From the abstract of the actual paper:


  We argue that Christie and Geis’s
  Machiavellianism scale (Mach-IV) is an instrument that allows the identification of types who
  vary in cooperativeness. We use that test to predict the behavior of participants in a two-person
  one-shot constituent game in which subjects face a choice between trust and distrust, and
  between reciprocation (trustworthiness) and defection. We find that the Mach-IV scale does
  not predict trusting behavior. It does, however, predict reciprocity. Over one half of those
  who score low to average on the scale reciprocate trust. High scorers overwhelmingly defect
  when it is to their advantage to do so.


In other words, high Mach's know when it's to their advantage to stab you in the back and do it. Big surprise there.

The game played, annotated with the results, is reproduced below.



I'll not give a full explanation (read the paper if you are unfamiliar with game theory, it explains it "for dummies"), but suffice to say that the high Mach (H) overwhelmingly moved down as P2, i.e. maximized their own profit even if that meant screwing over the P1 player who was nice to them, whereas average-to-low (AL) Mach players reciprocated (about half the time) as P2 even though that's not the dominant strategy as P2. In other words, AL felt like reciprocating for a past favor, even though that buys them nothing in this game. High Machs (mostly) did not repay a favor in such circumstances.