Would vide games rot your brain or would it make you smarter

It depends on the individual involved, the hours dedicated, and the nature of the games. It has been proven that playing video games can increase reflexes, knowledge, confidence, etc.
However, excessive time spent on this exclusive activity can certainly be harmful to your health, in the same way that concentrating on a TV screen for extended periods could cause headaches and eyestrain.
In general, there are games which are casual entertainment, educational, hardcore (requires mastery through time and effort to be rewarding), and many other gametypes which have different effects on the player. The individual will be affected differently by game content depending on how easily they get lost in something, and how malleable or gullible they can be in judging fact from fiction.