Gu techniques might be good, spells might be otherwise, but inherently they should not be inherently good or evil.
As long as one's intentions are just, learning Gu techniques does not mean one will go about killing the innocent at will.
To prevent this Mother Gu from escaping, the best method would be to capture it. However, Qin Niu did not possess the Gu Control Technique and had no way to confine it.
Now, the only method to deal with the Gu was to have Termite enforce spiritual control over it.
Or to simply kill it outright.
Qin Niu was considering whether to find another insect to devour this Mother Gu.
The next candidate would undoubtedly be the Green Silk Worm.
Having now evolved into a Mother Worm, its level was already high, and its capabilities were very akin to that of the Gu.
Having the Green Silk Worm consume the Gu would likely be of great benefit to it.
But in his actions, he always preferred to maximize benefits.
