A lot has been said about Tron’s trauma as depicted in Legacy and Uprising, and yes, he’s certainly been through a lot of traumatic and scarring experiences in those stories. However, on my most recent rewatch of the original film, I realized that even back in the ENCOM system, Tron has been forced to do things he might possibly have regretted later. He might be able to justify derezzing the MCP guards to himself since they are “enemy combatants”, but when we first see him, he’s in the arena of the Game Grid, and as far as I can tell, the opponents he’s throwing his disc at and derezzing are prisoners just like him, forced to fight just like him. Some of them might have chosen to join “the warrior elite of the MCP” as Sark puts it, maybe for power or political conviction, but possibly also because it’s the only way they think they can survive. Tron, meanwhile, is still defiantly holding on to his faith in the Users, and is therefore punished by being forced to kill his fellow Programs.
When Flynn realizes the serious nature of the game he’s participating in, he refuses to kill his oponent. The only reason Sark doesn’t derez him for this disobedience is because he’s been told by the MCP to keep Flynn alive, to prolong his misery. Tron doesn’t have this luxury, at least not as far as he knows (the MCP might want to keep him alive in the hope that he might eventually convert him to his side, but Tron doesn’t know that). Being put in the same situation as Flynn, I don’t think he would hesitate for long in “finishing the game”. To put it another way, if they hadn’t eventually escaped, it’s entirely possible that Tron would have been forced to face Ram in combat, and then what? Is that why he’s so grim and standoffish to his fellow prisoners? He doesn’t want to become friends with them, because then it would hurt even more when he’s forced to kill them to survive? How long would his faith in the Users have lasted if Flynn hadn’t come along?
So if the Tron in Legacy and Uprising is the same Tron as in the original movie (putting aside theories that he’s a copy, but even then it’s possible he retains the memories of the original), he had some dark memories even before Clu’s betrayal and all that followed. If Rinzler has Tron’s consciousness in the back of his head, I can imagine that when he’s in the arena, it’s giving him flashbacks to the ENCOM days, forced once again to fight and kill innocent Programs for the entertainment of a monster.
