A fully upgrated Cracking Attack gives you a 12sec buff if the attack connects, but if the attack results in a killing blow you don't get the buff at all.
Same thing about slaughter upgraded with laughter. If the attack results in a killing blow you dont get the laughter buff to melee power.
I don't know it's a bug or if it works as intented but i would love to see a fix so you get the buffs even if these two attacks result in a killing blow.