Conan...had to trust Kaito. Simple as that. Kaito was too good at hiding, told too many lies, played too many tricks. There was a point where he had to treat the other with practically blind faith that he would never change or he'd never be able to sleep at nice.

The interesting phrasing was probably just over exposure to Kid himself. "It's a scary thought. Anyone might turn out to be a bad guy if he could."