Because Theresia betrayed the doctor.

The doctor is the standard-bearer who controls the entire war. Everyone other than him is a chess piece. The doctor never cares about the life or death of a chess piece. He only cares about the equal opponent opposite the doctor. Kelsey's evaluation of the Doctor is: a ruthless war machine who has never been trusted. If the Doctor was still the same Doctor, Amiya would even consider putting the Doctor back on the coffin board.

Thearesia:

She is gentle and friendly to the people. She is willing to call the mercenaries by their real names instead of their code names. She is very loved by the people of Kazdaele, even Trey Some of the Sith supporters also had a favorable impression of Theresia.

In the event story "Born in the Night", Theresia led members of the Tower of Babel to assist W and others while W and other Sakaz mercenaries were escorting the Rhode Island, and the wounded W, Ines and Hedley arrived on Rhode Island.

Afterwards, W joined the Tower of Babel to serve Theresia out of respect for Theresia, while Hedley and Ines continued to cooperate with the Tower of Babel as mercenaries. . A few years later, due to betrayal by traitors within the Tower of Babel, Theresia was assassinated by Teresis' men. Her death enabled Trexis to win the Kazdaele Civil War and lead to the disintegration of the Tower of Babel Organization.

At the end of Chapter 7, from the conversation between Kelsey and W, it was revealed that the doctor was closely related to Theresia’s death, and that he had been hated by W for a long time.

In Chapter 8 of the latest main story chapter "Howling Light", Kelsey personally told the doctor that the doctor's hands were covered with Theresia's blood. It is not clear whether this was an exaggeration or a narration of facts, but only The Doctor has what it takes to kill Theresia.