I will never not get annoyed over people saying that Brian/Grue joining the Heroes was the obvious and easiest choice out there. Admittedly, the WOGs expand more on it, but even with what we get from Arc 4 when he talks about his trigger event, I can't comprehend how anyone wouldn't see the reasons for him not wanting to join the government sponsored heroes.

Basing this off memory, but to sum it up, Brian goes to him moms house after getting a text from Aisha asking for help. He finds her in some poor condition ( I don't think he goes into detail and it may not matter if he did cause he's not telling the full truth here) and when he tries to take her back to his dad's, their step-dad gets in thew ay and doesn't let them leave. Brian proceeds to beat him up, take his sister back, and gets powers sometime in between.

Then, he, aisha, and their dad go to the police the very next morning. They explain what happened and Brian still describes how he almost got sent to jail. Only aisha backing him up and his step-dad missing an asshole anonymous meeting saved him. And he still got some months of community service. Like, in my opinion he did the right thing every step of the way and still almost got screwed over.

Sometime later, (not immediately cause he's 14 to 15 when he gets powers) he decides to try and take care of aisha by being a villain to get enough money/backing to do it.

That's all described in arc 4. Now we get into the WOGs. Again, basing this off memory, but I can try to find them later.

First, Brian lied about his trigger, a little bit. General event still happened, but when he claimed he didn't know who the man was besides being his mom's new boyfriend, he lied. He knew that man bc it was the same boyfriend who had abused him in the past. And his mom brought him back and let him interact with his sister. Add in his mom's other problems and his dad's ("I wouldn't describe him as abusive") there was a fair bit of emotions Brian was going through that he didn't share. But at the end of the day, every adult in his life failed him and his sister. He wouldn't be willing to extend his trust as easily.

Second, a different WOG described Brian getting into crime in a panic bc aisha had done something afterward that led to her needing to get bailed out of something with money (again, going off memeory) so even if he wanted to join the wards he'd be doing it with a record. Presumably he later decided he might as well go all in on it and get aisha into a safer environment than both his parents. He probably thought it may help with keeping her out of trouble as well if she wasn't around two people who didn't know how to raise kids (not that he did either, but he probably thought he couldn't do worse).

Third, by WOG he did actually consider the wards, but after researching them decided they wouldn't be worth it. Admittedly this was moreso bc of his own hangups with commitment. Bc telling the big organization your power and identity then trying to bail wouldn't really work out, but apparently it was still preferable to a career of crime. It's not like the Protectorate would actually have authority to take a child away from their parents anyway without major proof, especially if that kid isn't a parahuman themself.

Fourth, the undersiders were generally a low risk high reward chance until Taylor joined and brought them her bad luck. They operated for a year and only got into some mild skirmishes with Lung and them being escape artist didn't have an issue with him. Besides that, they made a shot ton of money on top of the 2grand their sponsor gave them.

Fifth, I don't think we ever learned what the time table was for Coil helping out with Brian getting custody, especially given how suspicious Brian was of their mysterious boss and his motives preleviathan. Maybe Coil admitted to knowing Brian's problems and offered a solution at the start. Maybe Brian opened up with it as a condition to him joining. Or maybe after a few months of getting 2grand a relatively easy missions Brian decided to trust the boss who hadn't screwed him and ask for the money to be given to him in a legal paycheck for better chance at custody. I say that last option bc Brian is supposed to be the careful pragmatic one who thinks about his choices and doesn't do the crazy option unless bug girl decides to go full throttle and he begrudgingly goes along. And giving his personal details and a sure fire way of blackmail isn't excalty the pragmatic choice.

Tldr: he has to have some pretty big distrust of authority cause shifty parents and probably shotty police and by WOG became a criminal in a panic before going full in on it. Cause he was, again, like 15 years old.

Now, the argument for him joining the wards is that the Heroes/PRT could have snapped their fingers and given everything Brian would want. And seeing the deal Madcap got I can definitely see the idea behind it, but there are a few problems with that. A) Brian wouldn't know shit about how desperate the Heroes are bc they don't advertise that. B) I'm not sure how much power they would actually have over taking kids away from unhealthy environments cause laws and shit would probably limit that. C) as stated above, he says he almost got jailed for hurting his step-dad so I don't see him liking authority at this stage, no matter how just he was in the eyes of the law for it. D) Brian's own issues and that he's a 15 year old so he won't see the world as a perfectly rational being.