Wouldn't that imply that he fired the gun and then stuck it in his waistband? Was he involved in another shooting earlier in the night? I mean it's not likely he fired three shots, and was then shot by the cop and used his dying strength to wipe his hands off on his pants or underwear.
On the highlighted question - not necessarily that night. Your conclusion is entirely reasonable that he had fired the weapon previously, and carried it in his waistband.
Undoubtedly he didn't clean the weapon, either; gangstas never do. They buy dirty, cheap-ass ammo, too. He was more likely to have his pistol loaded with CCI Blazer or white box from WalMart, than Buffalo Bore low-flash HPs or Hornady Critiical Defense.