Diet Coke is actually not as bad for you as people think. It's sweetened with Nutra-Sweet, which is Aspartame, and although it was once thought that it could lead to cancer or other health problems, it's been disproven. There are actually no artificial sweeteners that are really bad for you, as Splenda has been targeted too recently, which is actually not harmful either.
The only thing that's bad about energy drinks is I suppose the calories. B vitamins, which are in energy drinks, are fat soluble, and you actually can have too many fat-soluble vitamins. Water soluable vitamins are pissed out when you have too many, like Vitamin C. Fat soluble vitamins are broken down by fat, though, so you can have an excess amount in your body. Too many vitamins can be bad for you too, but since there aren't a significant amount in drinks, you'd have to drink absurd amounts of energy drinks. Also, caffeine and things like that aren't particularly good or bad for you depending on how you see them. You can build a tolerance to caffeine, and it's a diuretic so it can lead to dehydration if you drink it constantly. It's nothing too bad, but it might give you a headache or make you feel shitty.
They aren't super bad for you, but like everything in life, they are only bad in excess.
