True. I had Payback in there because I thought to use it against a faster opponent before I mega-evolved, but you are right that Night Slash or Sucker Punch would be better (probably go for Night Slash for the guaranteed damage against an enemy lead). That said, I still like the idea of Payback to punish people from switching out, which would cause the double damage on the pokemon switching in. Alternatively I could take mean look as my fourth move to make sure I could take out the enemy lead.
Payback's power no longer doubles on a newly switched-in target. It's worthless now. Also, Knock Off > Night Slash for the utility of removing items. Not only that, the power difference between both is minimal. Refer to the Swords Dance set I posted.
As already mentioned, Absol isn't exactly suited as a lead, and it's better off using either Swords Dance or an all-out attacking set.
If you want an Absol with optimal coverage, here's the set:
-Sucker Punch
-Knock Off
-Play Rough/Superpower
-Fire Blast/Ice Beam
Nature: Naive
EVs: 252 Atk/4 SAtk/252 Spe
Item: Absolute
Sucker Punch and Knock Off are there for Dark STABs, the former for picking off frailer attackers, and the latter for a reliable utility STAB. Play Rough hits Fighting and opposing Dark Pokemon. Superpower hits Steels harder. Fire Blast also hits Steels, while Ice Beam deals with bulky Grounds.