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.

That said, the best user experience does not always derive from the perfect game. What the developers have to do is compare the experience for a new user (who knows nothing of Gandhi's warmongering history in the Civilization games) to the experience of someone who is familiar with the series and might even expect such behavior.

They clearly couldn't keep the old rule in place — that Gandhi suddenly gains the highest aggression value possible is unexpected for new players. Instead, they simply made Gandhi have a slightly elevated aggression — something that new players can handle ("Oh, haha, it's funny because it's Gandhi I guess") and something that old players will recognize. It's a good middle ground. (Note that they have kept around Gandhi's elevated aggression in more recent incarnations, though they have reduced its effect somewhat.)

But, again, from a pure gameplay perspective, the bug should have been fixed.

Kind-of. Civ 6 makes him sneaky and shitty aggressive rather than just straight-out homicidal. This matches my experiences pretty well: https://kotaku.com/gandhi-is-still-an-asshole-in-civilizatio...

So, I learned to play the game without learning Trade, and instead just destroying my enemies as fast as possible without it.