So today I tried one of those zero carb energy drinks. It's only 20 calories. I don't really plan on drinking them regularly, but I am just curious are they that bad for you? Are they any worse then a cup of coffee? It has 240ml of caffeine, 2kmg of something called Taurine.

In other words, it's chock-full of stimulants! The caffeine is just a bit more than you'd find in an over-the-counter (OTC) tablet like Vivarin and the like. Taurine is a stimulant found in all of those drinks like RockStar, RedBull and the like.
Zoom, zoom!

Once in a while probably isn't harmful.

It's about double the bean juice (though wide variances there) and Taurine is of dubious value. Personally I view these things as evil. If I want to perk up I'll just have coffee, which considerable research has shown to be beneficial in moderation.

Here is paper from a UCLA researcher that suggests that most of the energy drinks' effect is from the caffeine, and that taurine does not have an effect on its own or in combination with cafeine:
https://repositories.cdlib.org/uclabi...ol9/iss1/art6/