You're forgetting a key component of the story. This took place during the Great Depression. In rural parts of the South, unemployment was well above 50%, people were going hungry, and some were even starving.
This caused many men to do things they wouldn't normally do just to feed their families, like commit robberies, and even armed robberies. I have the feeling this is what happened to Del, a robbery gone bad.
Getting back to Arlen, he was a Native American. They are known to have serious problems with alcohol. If you add dire poverty to drinking problems, its not hard to imagine how a good man could go down the wrong road.
Wild Bill was really the only truly bad man on the 'Mile'.

Older folk look back at certain times years ago and only begin to realize that those times were happy times later, particularly within the context of what's going on in the present.
Arlen had a young wife and was in love and really didn't appreciate it then, not completely. We can't just speculate that he left the mountain that summer and ruined his life immediately. I haven't read the book but I'd guess that Arlen and his wife drifted apart over the years and his crimes occurred much later in his life.
Some of my fondest memories are those of when I was a child, but does any child stop and think how great those times were at that age?
Is very bad to steal Jobu's rum. Is very bad.