He went out to his car and retrieved his firearm. The question will be asked, if you were concerned for your safety....why did you return to the theater?
This isn't insightful, it's conjecture.
We don't know that he left the theater to retrieve his firearm, he may have had the firearm on him the whole time and had gone to get the manager... I find it far more likely that he was already carrying his concealed firearm vs. that he was leaving it in a vehicle and went to get it. The whole point of carrying concealed is to defend yourself, the firearm does you no good out in your car and it only becomes a liability if your car is stolen or broken into...
We also don't know that he felt threatened before he left, or when he returned.
From the story so far it doesn't sound like he walked back in and simply shot the younger guy, it sounds like he came back in, sat down, an argument or fight started, and THEN he shot the guy. If the guy was concerned for his life/safety it will be what happened after he returned that caused the concern.
Surveillance footage and testimony from witnesses will start to put this together. We'll know where the older guy went when he left the theater, and we'll start to get a better idea of what happened after he returned and potentially some insight into what could possibly have escalated this from an argument over texting to a fatal shooting.