Well could be something else... but based on what little info is offered that would be first assumption I would go on.
You don’t decide to kill you life long friend, let alone in such a public setting, without something pretty passionate at its core.
Turning a gun on someone at a place where most folks are also armed isn’t exactly a plan of a well thought out plan.
It could be that the shooter suffered from PTSD or other mental health problems. His lifelong friend may have seen him slipping away into psychosis and wanted to try to help him. He knew his friend liked firearms and wanted to connect to him through that activity. He may not have wanted to hang one-on-one with him so he planned this trip to the range as a way to have a group activity to do and have a positive impact on his friend.
Unfortunately, the shooter may have been too far gone and snapped due to the voices in his head.
I had a friend and a cousin go nuts. Sometimes, you see someone on their way into psychosis and you don’t know if the way they are acting is just a temporary low spot or them actually keep it together for a few minutes and they are even worse than you think. You want to help them by spending time doing “normal” things with them.