"...I'm starting to worry about Starswirl. He has not left his study for days now. He has all his meals brought up to him and a cot has been set up for him to sleep on...though, I think he has not slept these last few nights. He just keeps going through all the old texts and scrolls about the old legends and myths, in particular, the ones about that chaos spirit, Discord. He's planning something. I know it, but he won't tell me anything. He just keeps saying that it will 'bring about a new golden age', but I fear that my mentor will bring about the opposite..."
---Clover the Clever in correspondence to Commander Hurricane.
I've seen a few make the same accusation because and because of a line in Celestia's song where said something to the effect of "Starswirl didn't understand friendship."
I also once read a fanfic where Starswirl was outright evil and plotting to have unicorns enslave the other two races until he was stopped by a newly-ascended Celestia--who had only ascended because Starswirl tried to kill her while she was still mortal.