Simplify the following text for a Grade 9 native English-speaking person.
Original: FilipoviÄ became Chief Guard of Jasenovac concentration camp where he was nicknamed `` Fra Sotona '' , and he was given this nickname by Croats themselves .
Simplified: FilipoviÄ became Chief Guard of Jasenovac concentration camp where he was nicknamed `` Fra Sotona '' , and by Croats themselves .