What we know
: He has his face burned since he was younger, trying to defend his family from a firebender. ( if true )
He knows how to '' energy bend ''
oh and he is the leader of the equalist group
The reasons why he so angry agaisnt the benders, can be his case revenge or jealousy of that power ( if his history is based true ).
In both scenarios, his scars comes from an '' unbalance '' between weaken forces and overpower. Just like the firenation and the rest of world, the air nomads and the fire nation, etc.