Here, he's a fucking moron and doesn't merit an ounce of sympathy.
Ultimately, he's a fucking moron in both versions. In the books, he married some girl because he slept with her, completely ignoring the fact that he swore an oath and Frey men fought for him - and forsaking your oaths is not a very honorable thing to do IMO. Just dumb stuff a teenager might do.
In the show, he married because of True Love. And he also slept with her. Same result, slightly different dumb reason.
At least what Catelyn did was sort of justifiable. Robb is a dumb teenager in both versions, who does dumb stuff for completely dumb reasons, ignoring all common sense and logic. His downfall was almost entirely his doing. Too bad lots of good men paid with their lives because of his stupidity.