[@Vec]

Uhh it's a bug. The devs kept it as a joke.  This quote is from Kotaku:

In the original Civilization, it was because of a bug. Each leader in the game had an "aggression" rating, and Gandhi - to best reflect his real-world persona - was given the lowest score possible, a 1, so low that he'd rarely if ever go out of his way to declare war on someone.

Only, there was a problem. When a player adopted democracy in Civilization, their aggression would be automatically reduced by 2. Code being code, if Gandhi went democratic his aggression wouldn't go to -1, it looped back around to the ludicrously high figure of 255, making him as aggressive as a civilization could possibly be.