Yes they could have easily been in denial and seen any signs as "boys will be boys." They gave him a gun and trained him to use it, I could easily believe they thought his words and behaviors were fun and all in the family values and ignored all the warning signs, instead seeing it as some version of the way they talk to each other about killing people they dont like or dont agree with, without actually thinking their kid would do it.

They are certainly culpable, but its because of their values, and because they armed their child with a lethal weapon, and because of their relationship with those weapons. It's possible they were encouraging him but it's also possible it was sheer denial and ignorance about what they are teaching their child and the impact it might have. Everyone should google "family christmas photo guns" to see their behavior in cultural context.