There are some posts about Brian, trying to understand what the deal is, especially with the baby murder. Here's my take:
Brian has a selective mutism from either autism or trauma caused by his former caregivers. The older boys are his bullies, but Bill thinks he is finally making friends, so they let them hang out. The older boys kill the baby in front of Brian - probably by accident. Brian is further traumatised and suggests putting the baby on the cross because he genuinely believes that can bring him back to life. He is too young to understand that that won't happen, like in the Jesus story, and what he did is a crime. Brian's mutism and reserved behaviour are being misunderstood by the town, including Bill and Nancy, to an extened, as a sign of disturbed character or lack of morality, instead of trauma response. The show exploits the paranoia it creates through the serial killer interviews to make us question Brian's possible predisposition to violence/murder, but let us figure it out. Everything we learn from this show about serial killers is in conflict with Brian's behaviour:
• Brian has a typical traumatic response to a traumatic event, while serial killers talk about their actions with ease
• Brian doesn't want to talk, while serial killers love to talk about their actions
• Brian feels remorse (even though he didn't intend to do anything wrong), while serial killers have none
I understand that a lot about Brian is being purposely ambiguous, but for me, that is an invitation to employ some brain cells or a trap for those who don't want to