I do not think it is inconsistent that he could kill 4 people, hide their bodies and then try to sell the car and hang the keys and title on a nail in the garage/barn.
He has been trouble and always got out of it because his parents could afford lawyers who made deals with prosecutors.
He bragged about his 'badness' and actually told people he has killed before. Only wannabes do this.
He most likely went to a mental health inpatient facility in lieu of jail as part of his plea bargain and when he got out, broke the conditions by having access to a firearm.
His ability to get out of all the trouble he has been in has given him the 'untouchable' ideation of invincibility. He thinks he is smarter than everyone and that people are too afraid of him to rat him out.
He thinks he is a Made Man, Mafioso style but at the moment, he is just a pudgy punk in a jail cell eating baloney sandwiches and relying on his parents money and lawyers to save his butt from a life in prison.
He is good for these murders, I will bet on it. And, I do not think his dad helped him hide the bodies. Dad is smart and would NEVER have left the missing boy's car in his own barn.