What does "even a desire to do good can lead to the dark side" mean?
If you do good for yourself, just because it feels good or right, you just know it, and you are absolutely, utterly selfish in that, then this desire is genuine. But then you also will not expect gratitude. That’s how parents care for their baby children. That’s love.
But if you do it for others, and you expect something in return, the pretense is the dark side. When you don’t get what you expect, you may turn ugly.