Despite its current state, mangled, the dragon snake wasn't dead.
Rex wasn't familiar with the Voidal Monsters and how these creatures operate.
He wanted to purchase the information about them but it cost him three million gold for the information—a price he couldn't pay for the moment. However, killing Voidal Monsters gave him gold so he would have more than enough very soon.
Until then, he has no choice but to be perceptive.
The pink snake earlier that was neutralized by the Golden Waxing Talisman was a separate entity.
It was the dragon snake's tongue.
Rex betted on that to make sure that the dragon snake was really inactive.
Of course, the System already confirmed that but seeing the power the dragon snake commanded from earlier—it was instinct for him to be cautious. Had it attacked, he would have no choice but to instantly use the Invincible II item, wasting his trump card.
