There has to be far more beyond this picture. No easy single factor to blame. The devil has multifarious doors that he can knock on in a life in an attempt to damn it, or at least to wreck the reflection of the Lord in it.
Most of the lad’s secret died with him and we may never know it. What evils did he choose, what evils were pressing upon him, that in the end added up to this.
They will probably check out his brain and can tell a lot by the damage, blood work, etc. Believe me, there is also a history of mental illness and dead animals from childhood.