It's a very unusual clause to include in a Will - that any beneficiary who is indicted for criminal activity is excluded from the Will. Was there some financial monkey-business with siblings and estate distribution?
Did the Will also state that descendants of Will beneficiaries can nullify their parent's interest in the estate if they are indicted for criminal activity? If so, Mangione just wiped out his parents' inheritance.
Luigi Mangione's friend said that he always had a darkness about him. That sounds like a longterm friend who saw something that made him uncomfortable - but that's pure speculation. I keep a cautious distance if I meet a friend who has a "darkness".
To shoot a complete stranger in the back because Mangione was on a "Mission" to make a "statement" with his unibomber-inspired "manifesto"? That sounds sociopathic.
He had no empathy for his victim. His mission holds a strong prejudice against the health insurance industry, so he used extreme violence to randomly eliminate that perceived threat. He believes that he understands a higher order, and he retaliates to restore injustice.
I think he has hate-related psychological illness that make him
dangerous for the rest of his life.