There are a lot of betrayal flags up, but this guy is believable. Think about it. He's primarily trained as a barrier master. He's very powerful, but he doesn't want to become some kind of demon lord. He has to use his power to protect others and get what he needs to survive (and feed his new friends). He needs a job. Not just any job, he needs a well paying job. He could try a rougher lifestyle, but he's not prepared to live off the land at this point. So he's itching for a job and he finds an opportunity to work for people who don't seem bad, even if the family has some issues and there are warning signs. You might think this is too much like his tragic past, but that's what it's like to be a bodyguard, right? You can curse him as a moron for failing to acquire survival knowledge and continuing to work as a barrier master where the demand is greatest, but if he were a job seeker in your country, you'd probably call him an idiot or pathetic if he went months without a job and refused an offer because something looked suspicious about the employer's relative.