A few comments:
-firstly, an assassin needs quite good gear to stay alive, so you will have to do lots of runs, trade, or make a more gear-independent character (spellcaster) as an itemfinder. But with some patience, some luck and some shopping you could do just fine.
-what you're missing is an attack skill to program on your left mouse button to replace standard attack. Most (if not all) assassins use Onslaught; I'd suggest maxing onslaught, ignore pain and ardor, but leaving hamstring alone.
-poison damage is useful in normal, but becomes useless in epic and legendary. Hang on to Nightshade and Mandrake for the crowd control, but place only one point in Envenom Weapon and none in Toxin Distillation (at the end of, or after, Normal that is)
-flurry of knives is an excellent skill to spread mandrake and nightshade with, but throwing knive itself is rather useless. I'd suggest placing only one point in it, as a prereq to flurry.
-drop open wound and anatomy completely. Bleeding damage sounds nice, but is near useless; you don't want enemies to live that long anyway
-one point in War Wind is enough, and I'd ingore lacerate completely
-flash power is not required, but it can be a life saver, especially for an untwinked build
long story short, here's what I would do:
build