Originally Posted by
Saekee
Killer is a morale bonus that does not stack with other morale bonuses such as Rapid slash (dagger, Vistani tier 1) quickstrike (staff, TA, tier 2). So if you are using either one, you do not need killer. Ultimately, though, doublestrike is far more valuable in heroics than in epics in that builds usually do not have it too high yet. Only ninjas get it insanely high at level 12.
In epics, especially legendary, a strong group will be making trash helpless and they will want an assassin to drop held mobs as fast as possible, so No Mercy, Crush weakness etc etc. That is your role--trash cleanup--also if you have the Legendary Goldcursed Dagger you will be making stuff helpless anyway so better to drop them fast when you need it most, toe to toe.
So in short, I would do Killer in heroics, No Mercy epics.