You aren't the same guy who saved Earth from the hellspawn - that was your father. Everyone urged you to follow in his footsteps and work for the UAC, so you did. That is, until you noticed some pretty questionable stuff going on.
Rumor has it that in the bowels of the UAC they were building machines.
Strange and evil ones, the most dangerous of which is an exact replica
of the Cyberdemon your father encountered on several occasions. You don't
have any idea what they want with a Cyberdemon, but you don't like it.
Upon asking your boss (who of course is none other than the president of UAC; you hold a very high rank thanks to your family name) what it all meant, he laughed and told you to think nothing of it. "How do you like our new name, by the way?" he says to you, changing the subject. "Worldwide Aerospace Research corp. - A.K.A. WAR corp."
"I don't like it at all," you say, "and this is just proof that your little experiments have gone way too far."
"I'm telling you," he says, all humor gone from his voice, "don't pay any more attention to the experiments."
"You think you can keep this knowledge from the public? I doubt it. I'm telling you right now: I quit!"
You storm from the room, but you hear the president's voice from inside: "You won't last long out there! Trust me - your days are numbered!"
Of course, you realized that they would try to kill you for leaving the company - but if you're half the hero your father was, there's no army guy in the world who can mess with you. By laying low for a few months you manage to escape detection, until one evning a knock comes at your door. You are relieved to find that it's not anyone you know from the former UAC. It's two men who happened to be friends of your father after the "war." "There's no time for small talk," one of them says. "WAR is mobilizing its robotic creatures now. We have to take them out."
"And how do you propose we do that?" you ask. "You know as well as I do that almost all weapons have been taken by the UAC... before they changed their name, that is."
"There's one weapon that they didn't take. You know which one. And we know you can find something better as you go."
The other man hands you a slip of paper. "Here's the address of WAR's new underground entrance. You have 24 hours to prepare." he says. Then they leave as quickly as they had arrived.
You stand in the doorway, numb, as they drive away. Finally you glance over to your mantle, where on display is the one weapon UAC did not steal - your father's pistol. "Screw that." you say to yourself. "Better to surprise them now."
You grab the pistol and head towards the entrance to WAR, confident that you can face down anything WAR throws at you...