I think i've figured out who The Kid/Bill Skarsgard is
I think he's the real Henry Deaver.
Cause one of the the paintings from the Warden's basement had him wearing the same shirt Henry was wearing in the missing poster and its signed 1991 the year Henry was missing. The Kid starts telling Molly all about her childhood and says he knows cause he was there, and in a previous episode he was playing the same piano song from Henry's dads church service that we saw Henry at in a flashback.
Plus when we first meet The Kid he does infact say his name is Henry Deaver when the new Warden asks what his name is.
Maybe The Kid/Bill Skarsgard was some innocent by standard in 1991 who stumbled upon Henry when he was kidnapped in the basement, and maybe Henry's soul got swapped into his body thus causing it to stop aging.
Cause in a quick glimpse from Henry's head while he's in The Filter it shows Bill Skarsgard in a suit with his hair combed back walking down into a basement and turning around and screaming. Now i'm thinking Bill Skarsgard was a lawyer back in 1991, which is why Henry is a lawyer in current day cause Bill Skarsgard/The Kid got put into his body back in 1991.
But i'm sure you're asking, whats the deal with The Kid/Bill Skarsgard's powers to bring out everyones pain and cause death? Maybe the real Henry was evil or had some evil power to bring out peoples suffering, which is why he was kidnapped in the first place back in 1991 and why his dad was probably being told by god his son is evil. Henry's dad did tell Pangborn his son pushed him and caused his accident.
I'm sure theres a reason why it didn't show who was in the trunk in the flashback where Pangborn opened the Warden's trunk when he pulled him over. I'm thinking in reality it was little Henry as a kid but to Pangborn it looked like Bill Skarsgard. Perhaps that was after the body swap and The Warden was moving him to the prison basement.
I'm not sure all those facts add up but i just know the general twist is gonna be The Kid is Henry from back when he was a little boy in 1991, he hasn't aged a day since he was kidnapped, he's in an older body but his mind is still that of a kids which is why The Kid acts so weird and childlike almost.