The evil Furby is mean, and its real purpose in life is to take over the word. Once your plush is angry, it will refuse to keep food in its stomach, pretend it doesn't like to be patted on the head, and laugh like the most evil villain of cartoons history.
How do you get the evil Firby? If you spend to much time feeding your robotic pet with lots of food, it will eventually turn into some kind of hunger rage. For the quick hack, you can just push its tongue button a few dozen times during 30 seconds and then wait 10 seconds to see what happens (rinse and repeat if needed).
This personality is clearly the bad Furby, and is quite hard to please. Most of the time it will say "Me no like" when you pat it on the head, while a few times it will say "Me like"... It will often tell you how hungry it is, but will throw most of the food you give it. Feeding it with the iPhone app seems to work better than using the finger and the mouth.
You can watch the Youtube video below for to see how it looks like.
As you can see at the end of the video, the evil Furby is still angry when it falls asleep!
Please note that while your little plush robot looks angry and mean, it is doing so in very cute kind of way, a little like Pinky and the Brain in your childhood cartoons. For example, you can still tickle your toy and see its evil laugh every time.
Since feeding your toy robot is one of the main interaction when you are testing the iPhone App for the first time, it is actually quite likely that this is the first personality you will get after the baby phase. It is overall quite common to see your Furby turn evil during the normal use.