Well. I know little about demons, but Yeerks are people as well as parasites. Many people of many species are willing to cause harm to others to benefit themselves. A Yeerk has a very intimate view of the suffering it causes its host, and some cannot bear it. They balance this with their desire in different ways.

Cassie is an idealist who always seeks to understand other people. [There's a very careful neutrality to Toby's tone. Sometimes she admires Cassie, sometimes she finds her impossibly naive.]

Yeerks are living beings with physical needs that they meet in their pools. If one cannot leave its host for the pool every three days, it dies. This is very hard on the host. When something like that dies inside of you, you are changed. My people, when we capture a Controller, offer to kill the Yeerk quickly if it leaves early. If we can make them understand that they will not escape, some accept this offer; they consider starvation a death by torture.