I would agree, if that was all that happened.
Let's be clear, at no point did the old man ever brandish his weapon. He held it defensively. Pointed towards the ground, the entire time.
This other guy threatens to kill him multiple times, promising that he isn't joking, and will actually murder them, over and over. He then grabs a weapon, a weapon capable of murder, and moves toward them.
This is simple. Did they have reason to fear for their lives? Any sane person should answer yes. If your answer is yes, then it is a legal shooting.
Call his anger justified all you want for the old man pulling the gun, it doesn't excuse that threatening murder, grabbing a weapon, and charging a person, makes it legal to shoot that person.