You have this feeling inside you that you can't stand the sight/sound of someone - you can't be in the same room.
Then again there's animosity and there's loathing. Animosity is more violent dislike, loathing can stem from fear. Blind hatred is usually animosity, whereas loathing is the secretive version, cold, calculating. You don't let the subject of your hatred know they're 'in the firing line', whereas with animosity they're only too aware and any wtinesses are likely to point at you if anything untoward happens.
It's like the difference between manslaughter and murder.