Curse shouldnt be in your rotation, because you sacrifice way too much for too little return. say you drop hell blades for it, and you dont charge curse. so instead of doing 100~ damage to your target, you lower his defense by 2 and damage by 7 for 10 seconds. that doesnt seem like a bad trade off, except for the fact that curse is a 40 second cool down. in those 40 seconds you can get 600~ damage from hell blades in. DR.K. is not a hard enough fight to use a sub par skill, and he is doable at 25 with using under 5 blood packs.

id go into detail on my pvp build, but i feel that would bring up too much controversy. basically, use what feels right to you. do research, try things, test things out and see for yourself what works.