Hate each other.

It's rather all encompassing of everything they do but when you take all the terrible things people do to each other and break it down to the cause of it, you'll find that the common cause is practicing hate.

Hate causes war.

Hate causes jealousy which causes war.

Hate causes violence.

Hate causes abuse.

Hate causes mental destruction.

Hate causes bullying.

Hate causes sabatoge.

Hate causes heartbreak.

It's the root of all the problems our world faces and deals with everyday. If people would just treat each other kindly and spread love instead of bigotry and hate, the world would be a better place.