Happy face

Scientists were experimenting with different chemicals to try and find something they can use to make humans constantly happy, and barely feel any negative emotions.
And they've done it.
They've tried it out on one human, but the experimentee had attacked the scientists after being injected.
The experimentee was put in a help center after that.
But they've escaped.
And all they want to do is make others like them.
Always happy.
A mindlessly happy zombie.