Complex sentence: He was seen by the police on the scene as nervous, highly strung and volatile and they feared that the more pressure that was put on Naturale and Wojtowicz would cause them to be frightened and unbalanced and impossible to negotiate with on reasonable terms.
Split into simpler sentences: He was seen by the police on the scene as being nervous, highly strung, and volatile. The police feared that as more pressure was put on Naturale and Wojtowicz, it would cause them to be frightened and unbalanced, and they would become impossible to negotiate with on reasonable terms.