Since I don't know Bob I have no idea what, if any, mental conditions he suffered from. But poor decisions on his part wound up putting him in a vulnerable position- and someone was able to take advantage of that position. I think he trusted an individual, or individuals, who he shouldn't have. Sadly, it was a deadly mistake.
It's like, if you're overly trusting and let strangers inside your home you're more likely to be harmed by a stranger than say, your neighbor who won't open the door to anyone. Prostitutes are always being targeted because they are in a precarious position. It's too bad we have to live in fear, but these things happen all the time.