What happened to Bob and what is his relation to the plot?

First of all, Bob Taylor was once kidnapped by Mr. and Mrs. Jones, in the same way Alex was (and Anna and Joy for that matter). But Bob achieved to escape before getting killed (Mrs. Jones even mentions completely forgetting about him, until he appeared in the news recently). The reason why he is so deranged in general is just because of the traumatic experiences he had back then. It is hard to explain in particular what his actual insanity was, especially since I'm absolutely not an expert on the topic, but basically the kidnapping left some deep scars in his soul from which he never recovered (in the same way like Alex isn't really to call "normal" either).

In the same way he is obsessed with mazes because Mr. Jones, who once kidnapped him, was. Bob was himself presented with this strange imagery and sentences like "if you solve the maze you can leave" when under Mr. Jones' possession. And once he got this discredited book "Finding the Invisible Man", written by an FBI agent about a mysterious maze-loving kidnapper (and while it's not completely clear if the book actually was about Mr. Jones, it's definitely connected to him), into his hands, those memories all broke through and he started to recreate those kidnappings, maybe up to the point of pretending to be this maze-kidnapper himself.

As a result he worked himself further and further into the fantasy of kidnapping children himself. That is the reason he bought/stole children's clothings, smeared them with pig blood and buried those boxes full of dolls in his garden. He was imitating the "Invisible Man", pretending to have kidnapped/killed the children, even if not ever doing it for real. Maybe this even went up to the point of where he himself believed to have actually done it for real, which might be the reason he shot himself in the interrogation room (though, that might also have had other reasons, like a general discontent with his soul-torturing life).

As to the snakes, I'm not sure their connection to Mr. Jones or the "Invisible Man" was ever mentioned (but it wouldn't be too far-fetched, given the many religious motifs throughout the movie, and especially in Mr. Jones' motivation).

So to sum up, Bob Taylor didn't ever participate in any kidnapping, apart from his own, where he was the victim of the same kidnappers that got Alex, Anna, Joy, and all the unknown others. And it was that kidnapping that put him in a deeply deranged state and ultimately drove him to play through fantasies of kidnapping and child murder, though never doing it for real. He was thus in some way a red herring/wrong track, while on the other hand still being connected to the actual kidnapper and ultimately helping Loki to figure out the whole picture.

Well, Taylor is an adult when all of that happens but he was a victim to the kidnappers when the kidnappers were younger and practiced more occult rituals.

He was tortured and tormented by them in ways more various and abhorrent than the drugged soda pop and mazes the current two girls underwent.

So what we are seeing is nothing more than a broken person with severe trauma who is trying to live a normal life but can’t escape the years of systematic torment he underwent as a child himself, and those things he’s does are symptoms and reactions to that. It wouldn’t be going to far to state that those things he’s doing is his way of coping with what happened to him.

What the detective (I forget his name) failed to realize was that the criminal they were looking for may not have been young like in his 20s or 30s. If he realized that Bob was a victim, he would have had to expand the profile of the kidnapper to be much older.

But he didn’t figure this out because he didn’t inspect the corpse found in the priest’s basement, otherwise he would have noticed the maze necklace around the corpse, considered what the priest said about that corpse (he confessed to kidnapping children), and made the connection.

I'm writing this just to answer the "snake" bit. In the last scene of the movie when the CSI people are leaving there is this little chat between one of them and Loki:

Loki: You all done for the night?
CSI guy: Yeah, the ground's frozen solid. It's gonna take weeks to excavate the entire property. Just found some dead snakes and shit.

It somewhat points to the possibility that the Joneses were using snakes (and shit - whatever it is) to torture their victims (maybe by throwing them into the pit with the children).