If you don't think he should have done it the how can it be justified?
Self defense is in the mind of the person facing an attack. Once the text dude got up and turned around things escalated fast. The man was reportedly 6 4 tall and almost thirty years younger than the retired cop. I can see where the old guy may have felt threatened.
Feeling threatened is not justification for shooting someone. Florida's law states, "a person is justified in using force, except deadly force, against another when and to the extent that the person reasonably believes that such conduct is necessary to defend himself or herself or another against the others imminent use of unlawful force." The statute goes on to say that deadly force is lawful if a person "...believes that such force is necessary to prevent imminent death or great bodily harm to himself or herself..."
Reeves was never in dange of imminent death or great bodily harm. He was justified in throwing a punch maybe. But not in shooting an unarmed man, especially when Reeves had provoked the confrontation to begin with.
In Texas there is an exception to self-defense called "provoking the difficulty." It means that you can't provoke someone, then claim you feared they were going to attack you, and "defend" yourself with deadly force.