3 Reasons why Morality is not a herd instinct.

Moral laws or morality is not a herd instinct.

There are three ways to look at it.

Desire to help or Play safe: If you see a man drowning, there are two instincts at play. One says that you should save the person and another says that you should keep yourself safe. Both of these are instincts however in addition to these there is a third thing that tells you which instinct to follow. These instincts are like the keys on piano the keys itself don't determine which key will play at a given time. But the thing that decides which key to play at a given time can't itself be a key. 

Stronger Instinct : these instincts are not morality. Cuz if the two instincts are in conflict, and there is nothing in a creatures mind except those two instincts, obviously the stronger of the two must win. But that's not the case, we see many times that we allow the weaker instincts to rule. We try to help others even when it means us harm. The thing that tells you which note to play louder can't itself be the note. 

Good instinct and Bad instinct: If the moral law was one of our instincts then we could say that one of the instinct is always good and the other is always bad. The most dangerous thing you can do is to take any one impulse of your own nature and set it up as the thing you ought to follow at all costs. There is not one of them which will not make us into devils if we set it up as an absolute guide. You might think love of humanity in general was safe, but it is not. If you leave out justice you will find yourself breaking agreements and faking evidence in trials ‘for the sake of humanity,’ and become in the end a cruel and treacherous man.

Source: Mere Christianity by C.S Lewis