I don't understand the concept of this series (i.e. Voight's "deal" with IA)

HI,

I didn't watch Chicago Fire but from some online search I've understood that Voight was a piece of work in the past. It's not clear to me if he was really a "full fledged" dirty cop in the classical sense, or if his "only" sin was to threaten to kill someone to try keeping his son out of jail, which objectively is a huge no go and it's understandable that IA has such a bad opinion of him.
I too can't quite feel respect for him because of that, despite the fact that I'm a big fan of morally grey approach to fighting crime and I appreciate this side of him.

Anyway, why do BOTH handlers (the b... in the first few episodes, and then Stillwell) hate him so much, so unconditionally?
I mean, if that's the only wrong thing he ever did, why so much hate?
The b... is a climber, I get that, she doesn't care about truth or fairness, she just wants to get dirt on him so she can look good. But this doesn't justify her intense hostility.
And Stillwell seems to have a personal hate like if Voight did something very bad to him.
He even killed a policeman pretty much only to have something to frame Voight afaik.
Why?

Other than that, I don't understand the deal.
Why would IA care about making a deal where he has to catch high profile criminals? It's not IA's realm.
It seems like complete nonsense to me.

Btw, he not killing Pulpo was a MAJOR disappointment.