Even though Loki didn't know what the danger was or why a defeated foe was dangerous to him, he still sensed danger, so he sought to eliminate that danger as soon as possible.
After Loki heard Time Traveler's last words, he heightened his senses to perceive anything that might be out of place. But he couldn't sense anything, and nothing went wrong until Time Traveler was sealed.
With the black force field trapping Time Traveler, he couldn't escape. It seemed the seal also prevented him from exploding the Supreme Metac on his way out. The Devouring Ability was also ripping into him while he was being sealed.
His lifespan was the first to go. Then his consciousness within the Supreme Metac followed after he lost his lifespan. So all that was left to be sealed was an empty Supreme Metac that was ripe for the taking.
