"I live to serve." He's said it in passing before, and while it could come across as snide, he means it. Doul might not live to serve Huaisang, but there's some sort of code that he uses as a moral compass.

Fortunately, as canon suggests, Doul and his team do succeed. They're able to tunnel, to remain hidden, and more importantly, find a rather useful dead body. How they're going to use that information still remains to be seen, but one way or the other it will bring an end to the conflict.