Slark is, in my opinion, best played as a more bulky type of hero. His ult can keep him safe and help him escape (or tank longer), his passive benefits him more from prolonged fights than quick skirmishes, Pounce is a fantastic initiate and escape, and Dark Pact deals AoE damage and purges most debuffs.

He has nice nukes and utility to go along with a bulky role, and can play it quite well with the appropriate items. Not quite a tank, and not quite a support, but somewhere inbetween. Your job is to be a nuisance in teamfights that just doesn't quit.