The first major red flag to me is a lone officer going to investigate a domestic disturbance. The second major red flag is when the killer agrees to come outside and then is allowed to retreat to his bedroom. Under no circumstances will I allow someone who is detained or under arrest to do that. If you're being detained pursuant to an investigation or I've told you you're under arrest for a warrant and you need to grab your shoes, grab your smokes, get a number out of your phone, call your mama, etc. that is fine that you need those things but you're going in cuffs first and then we will work all of that out. I put people's shoes on their feet if I have to after they're in cuffs. I have adopted that policy based on a personal experience from when I was new where I delayed handcuffing someone who managed to get away and also the experience of another officer who used to work at my department. The other officer's experience was that he went to arrest someone on a relatively minor warrant. The guy didn't have a shirt on and said he needed to go in his bedroom to put on a shirt. He came back out wearing a shirt but also had retrieved a gun. The officer was able to react quickly enough to shoot and kill the suspect. I would prefer to not have to go through all that. The other red flags from the video are trying to search from the side instead of behind and once the search started the guy was reacting in a way consistent with people who have something on them.
I am very very sorry that the officer was killed. When I think back to the way I handled some calls when I had only been on the job as long as him, I get chills thinking about how easily I could have been had.