I think that his other motive was to make it clear to ELiz that Wickham was not a good person.
He does not beleive that she's likely to marry him, since W would not marry a girl of no fortune. He certainly doesnt beleive that she woudl have an affair with Wickham, But he may well wish to put an end to her affection for Wickham, partly becuase he IS jealous and partly becuase he may feel that she SHOULD know that W is a bad lot, and that it may help her get over her fondness for him, if she knows waht he is really like. Then while she may suffer some disillusionment, she is less likely to be hurt if Wickham does soemthing really insensitive which will disillusion her, like maybe TELLING Her that he'd like to marry her but he can't afford to and he must now try and marry Miss X who has £20k. (or if she goes home to Meryton and finds that Wickham has seduced the Bennet's parlourmaid.
D is not sure of Eliz's exact depth of feeling for W so I think he jsut wants to tell her "look he's a bad guy, I dont like ot see you in thrall to such a person and if I tell you his misdeeds and you can beleive Im telling the truth, then I think it will hurt you less in the end and you will get over any feelings for him, without a public disillusionment.