The defense made it relevant. If the defense is going to argue that elements of Floyd's character and past are relevant to the case, then the prosecution generally gets to undercut and counter that with facts of his character and past that cast him in a more positive light. Similarly, if the prosecution gets to provide such things, then the defense gets to provide their own evidence to the contrary.
Furthermore, opening statements are not held to a "strict, dispassionate facts" standard. If anything, it is closer to say they are prohibited from this, and attorneys are actively encouraged to avoid dispassionate opening statements (studies have shown that most jurors know what verdict they will return as soon as the opening statements are done). While legal arguments are not permitted, courts have held that "enthusiastic rhetoric, strong advocacy, and excusable hyperbole” are permissible in an opening statement (Com. v. Siny Van Tran, 460 Mass. 535 (2011)).
And that pretty much covers your concerns. All you're complaining about is nothing more than stating "this man's death was avoidable, unwarranted, and unjustified", which is exactly what the prosecution is about, only with an extra helping of enthusiastic rhetoric, strong advocacy, and/or excusable hyperbole.