The worst thing you can do is get caught with the gun on you.
That's why most hits are done with stolen guns that can be dropped at the scene.In addition,if a guy is not wearing gloves,he certainly will make sure to remove his prints before leaving the scene.

If the cops can't tie you to the murder scene,it will be tough to convict you. Other than prints or witnesses,there's not much to go on.

If you put two behind the ear of some guy,the only DNA at the scene is going to be his. The chance of leaving any usable DNA of your own by simply pulling a trigger is remote at best.

That's why these guys drop the gun,go home and shower,and dispose of the clothes they were wearing,in case they got some microscopic traces of the victims blood on them.

Ballistics can't tie the casings or bullets found at a scene to anything but the gun used to fire them. That's why you don't ever want to be holding the gun.