Keep in mind that the CEO of United healthcare wasn't just hated by millions of people that he had wronged. He was in a position of power that I am sure other people wanted or wanted to see him fail.
The whole loan gunman with the message written on bullet casings is very cinematic. It may be to throw people off the trail of someone else who profits from his demise. His competitors were all more than Rich enough to arrange a hit like this.
Do I think this is the most likely scenario? No. But you have to list out all the plausible hypotheses and then adjust your probabilities as evidence for and against accumulates.