End is actually endangering others around him unintentionally. Whenever he holds in his instinct to attack whenever hurt, or stared at it builds up. If he keeps this up he could possibly have an outburst of violence to others around him. Immortal is aware of this so this is why he trains him, so in the fighting process he can let steam out.

Example:
End snapped at one of his friends because they shoved him a bit, he calmed down quickly though. Immortal advised him for training and he almost hit her while fighting.