Most D-I running backs are great athletes; what makes you think he will make a great linebacker? One running back translating into a great linebacker doesn't mean all will.
Castille just needs to take care of the fumbles. We're not going to have a lot of depth next year at RB. There's no sense in having only one RB next year with real experience.
Because most schools recruiting him were going to use him as a LB. He was rated higher as a LB than running back coming out of HS. Because he looks to hit people with reckless abandon and forgets he's running the ball sometimes therefore fumbling.
That what you want from a LB. Speed, intelligence, and agression.
However, let's see what Bo and Carl think about it. I am of the opinion that these two are very gifted defensive minds. It is like poetry when this defense executes, albeit rarely. Getting more and more consistent every game.