You take the grenade from Hank's vest and throw it into the vent. You and Hank then run as far away as possible before the grenade explodes, killing the Behemoth and causing a large portion of the wall to collapse onto him. You and Hank now move on to Subject Zero's lab. There, you find a canister next to a dead and heavily mutated carcass of a man with the nametag "Larry Garris". Garris, that's the name from the file.
Hank points at Garris's dead body and says, "Looks like someone tried to get here."
"No. This is the guy who was behind all this. That's what this was about. He wanted to restore his credibility. If he saved the city from a potential doomsday scenario, he would become honored among the scientific ranks. I found a file that included him earlier. He was the Behemoth's father, in every sense of the word. He was the father to the human that was to become the Behemoth, and he was behind mutating his son into it. He was fired, and he wanted to continue his research. This was all about the Behemoth, not Subject Zero," you say as the plot unravels before your eyes.
"Well, whatever caused this, I'm glad it's over. Mission complete."