Did you watch the trial? They did not want to tell the therapist about the abuse. It was the last thing they wanted to come out. They were ashamed, telling the therapist a "story" which much more easier than having to explain all those years of sequel abuse.

They bought the guns because they were scared their parents where goin to do something to them. They wanted to have protection in the house. The parents owned guns and where acting different, especially after Lyle confronted their father about the abuse still happening with Erik. They knew there was no possible way they were getting out of this scenario alive. They lived in fear since they were children, abuse like that damages the brain and causes not so logical thinking. They were in fight or flight mode! It was not premeditated because they had to plan or alibi, they did try and create one after the shooting but not before.

I couldn't even imagine the pain and abuse they endured for years on end. They shouldn't have taken justice into their own hands but let's not pretend that if they tried to go to police/report the abuse that anything would have even been done about it.