Gao Yuan took three seconds to make a decision, and then, he felt that the Serpent People were insulting his intelligence.
The Serpent People wanted to negotiate with Gao Yuan using the spoils that belonged to him. If that wasn't an insult to Gao Yuan's intelligence, then what was? What the Serpent People should have been doing at this point was kneeling and wagging their tails in begging, not attempting to negotiate terms with Gao Yuan.
Because the Serpent People no longer had the qualifications to negotiate peace.
Peace was fought for, not talked into existence.
Originally, humans had no right to negotiate, so the Serpent People ignored humans. Now the Serpent People had lost the right to negotiate, so Gao Yuan didn't want to bother with the Serpent People.