Ohh.... how did you make that awesome poison?
Imp Stool as a base, then add Human flesh, if you cant find human flesh you can substitute it for Canis root, the third ingredient can be either Slaughterfish eggs or scales, or orange dartwing. You can use Mora Tapinella but it will have the side effect of "restore magicka", which wont mean much when your target is dead.
With the pickpocketing perk, to successfully place powerful poisons you need strong pickpocketing. The stronger and more valuable the poisons, the more pickpocket you need to do it successfully.
Also if you want to greatly enhance the effects, administer a 'weakness to poison' poison first and let it take effect, this wont alarm the victim, then apply the lethal dose which amplified results.