In True Grit, why was Texas dude (Labouef) with the bad guys when Cogburn shot him accidently?

They split up, girl and Cogburn try ambush but theres scout...and then
huge group of bad guys and the Texas Labouef good guy! ???
Oh, while i'm asking, why did Cogburn shoot Blackie (horse) when it
couldnt carry them any more (at end of movie)?

And last but not least, why did Cogburn cut an "x" on girl's snake
bite on hand?

Thank you very much for answers