From what I remember, the Merciless card says that you get a 25/40% damage bonus to your next shot, after you've killed an enemy. From what I've tested, you can actually miss a few shots after you've killed an enemy, but your next damaging shot will have the damage bonus.
In addition, enemies that you kill with a Merciless enhanced shot, will not give you a damage bonus for your next shot.
For example, with my KLT-13 Doldren, a fully charged headshot at optimal range does 251 damage, and, with the Rare Merciless card, it does 351 damage with a headshot after a kill. However, if I kill an enemy with a 351 damage headshot, my next headshot goes back to 251 damage. After I kill an enemy with the 251 damage headshot, the next one after that does 351 damage, and so on and so forth.
I'm not sure if this is a bug or intended, as at least the way the card is worded, I would expect the damage bonus to keep carrying over from one kill to the next.
