I know he was insane because no sane man would do what he did.  But you may have a different definition of insane.

To some, the civil war was as much about state and regional pride as it was about slavery.  You can project your feelings as you wish and with guns you can impose your will.  However, without effort you will never understand what motivates people who are fundamentally different from you.  You think you are good and they are bad.  Thus you have no problem condemning them.  But this very process is the same one that "white supremacists" employ.  Perhaps you are no better than they.