Why he was so rebellious
Well , a very likely reason is post traumatic stress disorder(same thing Rambo had).PTSD is believed to be caused by psychological trauma such as war,abuse,stress,etc.
If for Niju,I think a childhood event such as loss of a family member would most likely what caused His fear of change because your past can effect you.
However, it is capable of being treated through various methods like theraoy and might e possible to be gotten over,which means that He could changed since Niju didn't seem like cruel guy like Steele,He could just be overwhelmed by fear.
P.S.for the military, It comes more in the army and marines(special forces like the green berets count as army)
If any questions feel free to ask,but please no bad comments