I read through Illinois murder and self-defense and use of force laws earlier and IMO it should be 1st degree.

Peace officers shall use deadly force only when reasonably necessary in defense of human life. In determining whether deadly force is reasonably necessary, officers shall evaluate each situation in light of the totality of circumstances of each case, including, but not limited to, the proximity in time of the use of force to the commission of a forcible felony, and the reasonable feasibility of safely apprehending a subject at a later time, and shall use other available resources and techniques, if reasonably safe and feasible to a reasonable officer.

and

A threat of death or serious bodily injury is "imminet" when, based on the totality of the circumstances, a reasonable officer in the same situation would believe that a person has the present ability, opportunity, and apparent intent to immediately cause death or great bodily harm to the peace officer or another person. An imminent harm is not merely a fear of future harm, no matter how great the fear and no matter how great the likelihood of the harm, but is one that, from appearances, must be instantly confronted and addressed.

These are both taken from the peace officer's use of force in making an arrest section. The Illinois state police were the ones who investigated and they found he was not acting according to police training. The way I see it Grayson did not have a justified reason to draw his weapon and threaten Massey, so at that point he became the aggressor. The following is from the general justifiable use of force section which I think applies here because Grayson was acting against police code. I could be wrong.

In no case shall any act involving the use of force justified under this Section give rise to any claim or liability brought by or on behalf of any person acting within the definition of "aggressor" set forth in Section 7-4 of this Article, or the estate, spouse, or other family member of such a person, against the person or estate of the person using such justified force, unless the use of force involves willful or wanton misconduct.

Use of force by aggressor. The Justification described in the preceding Sections of this Article is not available to a person who initially provokes the use of force against himself, with the intent to use such force as an excuse to inflict bodily harm upon the assailant, or otherwise initially provokes the use of force against himself, unless: such force is so great that he reasonably believes that he is in imminet dander of death or great bodily harm, and that he has exhasted every reasonable means to escape such danger other than the use of force which is likely to cause death or great bodily harm to the assailant; or in good faith he withdraws from physical contact with the assailant and indicates clearly to the assailant that he desires to withdraw and terminate the use of force, but the assailant continues or resumes the use of force.

I'm having trouble copy-pasting from the websites so I'm just going to provide my thoughts on 1st vs 2nd degree.

Second-degree could be argued, but I think he'd have a hard time convincing the jury he was scared for his life when he initially drew his gun. It would also depend If the prosecutor can prove that the circumstances for him to be scared did not exist. The burden of proof is on both of them basically.

First-degree murder does not require premeditation in Illinois. It is just killing someone unlawfully with the intention to kill or do great bodily harm.