To me it seems that Wickham was the sort of young man who knew his power with women, used it shamelessly, and having found yet another (Lydia) swept off her feet by his charm, took advantage of her innocence and immaturity and exploited it with nothing more than lust as a motive. Perhaps running away to London may have had something to do with the debts he has accrued (as evidenced by Col Forster) and again running from the situation and not facing up to the mess left behind. I can imagine Lydia being swept up in the "adventure" as she always wanted to go to London (according to Kitty). They are both shallow and irresponsible without much of a thought for the future, so I suppose it seemed like a good and fun thing at the time.