Bolin decided to prank Mako all week till the point that Mako was really angry at Bolin and they had a huge fight. Mako was a little bit too angry and didn't know when to stop.. till he burned Bolins left shoulder and left ear.. 3rd degree burns...

You walked in and saw Bolin sitting on his bed, balling his eyes out and Mako nowhere to be seen. He probably ran away when he realized what he did to his baby brother.