Yes, you should be ready to render aid to the perp if your safety is not put at risk by doing so.
I carry two standard GI pressure dressings in my vest, both for me if necessary; if not, I will use them on a perp if and only if I can secure the scene and the erp is not able to be a threat. The presence of the torn wrapper and the dressing bloodied and in place indicates I was not a man out to murder anyone, but that I called the police, asked for an ambulance, rendered basic 1st aid (or attempted to-the opened dressing would be on the ground near the perp when LEOs arrived). Also, I am an advanced life saver qualified Soldier, so I can demonstrate my ability to render aid as well.
The other question you present-SHOULD you render aid to a scumbag whom you just had cause to shoot; well, do not try to say that to a jury.....
God Bless
Molon Labe
In many cases, I would think it should suffice to say that one believed the person might pose a threat. Perhaps the 'attacker pretends to be bleeding to death so as to ambush the intended victim' thing happens more often in movies than real life, but unless one has assistance to secure the scene one could very reasonably argue that getting close enough to render first aid would be dangerous.