Now... What will Luigi Mangione say for his defence?
1. He is not the shooter and has nothing to do with that assassination.
2. He is the shooter, deemed the CEO responsible for "denials of claims" that led many to get sicker, die or face serious financial problems, and he chose to assassinate him as punishment and as a warning to other CEOs, the healthcare industry, etc. He therefore sees that act of planned, cold-blooded murder as "self-defence" of sorts 'cause he was defending the interests and very lives of the American people. He gave himself moral authority to bypass the law and commit "vigilante justice."
3. He was insane at the time of events and could not tell right from wrong.
If he argues the first point and it is proven beyond a reasonable doubt that he is indeed the shooter, it'll make him uncredible to the jury, and it'll show lack of remorse.
If he argues the second point, he'll be convicted unless one or more juror agree with such an act of vigilante murder, with his reasoning, and I'm sure the prosecution will do its darndest to eliminate all such outrageously biased jurors. I'm also certain the prosecution will do damnedest to humanize the victim, Mr. Brian Thompson, demonstrate that he was not so responsible for all the denials of claims, that his decision-making power was limited, and that he was actually working on cutting out-of-pocket costs, among other things.
If he argues the third point, it'll be an uphill battle for him and his defence team, esp. if he shows no actual remorse, did not have a history of serious depression or mental illness, and all the premeditation will not help.
In all cases, the prosecution will likely emphasize the impact the murder has had on the victim's friends and family.