Hmm..tis a shame that these ponies do not understand Discord's methods. You see...for you ponies the phrase "There is method to my madness " is what you all prefer to use. However, for the Agents of Chaos, the phrase is "There is madness to my Method."
Discover has no desire in conquest nor does wish to kill or maim or even torture. His sole enjoyment in life is causing as much chaos as possible. He did not teleport the Main Six because he wanted to play with them. Watching them turn against each other was more entertaining than he has had in eons.
If you pay attention, he is actually very fascinated with Twilight. I feel that she is his new favorite toy (much like I assume Celestia was his favorite toy before his original defeat).
Furthermore, what is so threatening about being turned into stone when you are immortal? He will escape eventually and to an immortal, 1000 years is basically a year to us.
Stupid? Not at all. While obviously surprised that Twilight was able to break free from his manipulation, it isn't like he was irritated or even angry about it. If he had been, he would have extracted revenge against them the moment he was released the second time. Furthermore, he used his 2nd release to destroy all knowledge of returning him to stone from the libraries and since he made Fluttershy swear to never use her element against him, he is basically free to do whatever he pleases.
Discord is very clever but he is not some Hannibal Lector. He is an Andy Kaufman. He causes chaos, he stirs emotions, and he trolls ponies for his personal enjoyment. Like Andy Kaufman, sometimes things did not always go according to plan, but he still has his fun as long as he is able.
Arrivederci!