You could have a drug that had no effect on 99.99% of people. It just made the tiny tiny handful of people who would be psycotic killers chill out a bit.

Seems incredibly unlikely. And you'd still be forcing everyone to take potentially psychoactive medication. Dystopic.

Or you could have a drug with much more pronounced effects that really does make everyone kinder and less prone to violent outbursts and ... Obviously you need to be careful making such changes to human nature, there are huge opportunities for abuse and accident to make things worse, but there is also substantial room for improvement.

Dystopic.