well, it's not that Berserker is just insane though. He literally traded his sanity and his consciousness for power. He's just not capable of complicated thinking. Remember that if he sees someone he definitely wants to kill, he will completely ignore orders and attack them.(in Fate/Zero he ignored his master's orders and attacked Saber every time he saw her.) And, there's also his Noble Phantasm called For Someone's Glory, which obscures anyone's sight of him, makes it so you can't detect how powerful he is, and makes him seem kind of like a hallucination because sometimes it doubles or even triples his appearance, so he looks like a hallucination or a delusion at times.