There's a lot of potential legitimate reasons that guy could've been there sleeping on the couch with a pistol in his hand or right next to him. If we are playing hypotheticals, how about this one: dead guy's sister lives in the apartment, and the suspect the cops are looking for is her crazy ex-boyfriend who beat her up the night before. She called her brother to come stay with her because she was worried he was going to come back and kill her. Brother, who is a CCW holder and all-around good guy comes over and does exactly that, hoping nothing happens, but bringing a weapon just in case. He gets woken up to a bunch of people coming in the front door shouting, and naturally grabs the pistol while he's trying to wake up and figure out WTF is going on. Cops see gun and kill him on the spot.
Now that probably isn't the exact scenario that played out, but it sure could have been. Without a no-knock warrant being served, that guy is still alive. Most people on this board sleep with guns readily accessible. If the police seeing a person with a gun in their hand inside a private residence (where they weren't invited) is justification to shoot them, we have a big problem.