I see your point, but this assumes Katherine's going to die/pay for her crimes.

I'm not sure she will be killed. It's a possibility, but I think the writers need her there to play out the triangle. That'll get old (LOST-alumnae here) but it's the more likely path than Stefan and Elena killing Katherine to get rid of her while giving Damon a chance to redeem himself.

Of course if Katherine dies and Damon remains the anti-hero, yes. There's a problem. But then, the show would be over pretty quickly.