If the truth will get him what he wants, he would have no trouble telling it. I think of a compulsive liar as someone who tells the lie for the thrill of seeing if he (or she) can get away with it, con the listener.

What Wickham really revels in is the half-truth, most readily demonstrated in the encounter with Lizzy at the Phillips house. He probes to find out what she knows or how she feels and then tailors his story (true, false, or a mix) to elicit the reaction that most benefits him.

(I once had a teacher who said that a half-truth was worse than a lie. I showed the teller lacked the daring to tell the lie but lacked the courage and moral fiber to tell the truth.)