This person was as capable of stopping a crazed killer any place, anywhere as anybody here. As sad as I am to say this, this man was killed by a person he trusted. This could have happened to anybody, anywhere. Whether it happened in a luxury golf club or in the middle of nowhere in the desert, it doesn't matter. If some crazed killer you trust pulls a gun on you and shoots you while you are not looking, you are as good as finished, usually.
There is a reason why serial killers are very effective at what they do. They are generally people you trust and who seem like reputable and respected people. These people thrive off of this image to find their victims when they are most vulnerable to attack.
Sadly, Chris Kyle put his trust in the wrong person. Anybody you go shooting with, is someone you trust your life with.
This is sad, but I think all the hypotheses and theories of why this happened is useless. It's time to face the music of life.
I really don't even know if there is anything that could be said. Maybe the only advice I can give is spend a month getting to know somebody before you go and turn your back to them, while they are holding a gun. If you are not willing to do that, then you will just have to face there is always a risk. Even I am not willing to do that, as if I meet somebody I think I like, I'd probably just go out and go shooting with him. Done it many times, with people who were still mostly strangers.
If a Navy Seal and hero like Chris Kyle couldn't have stopped this murder, don't expect that you easily could have. Nobody ever expects a friend or a person they trust to murder them.