I was creating a character in the Creation Kit and I added the character to the BanditFaction with the intent on changing the character's disposition towards bandits (in other words, to make the character be enemies with them), so I changed a lot of the relationships with other factions to "Enemy". This causes all bandits in-game to attack and kill each other. No big deal - so I thought I'd just change all the relationships back to "Ally". Unfortunately, this had no effect on the bandits. They still kill each other. I deactivated my plugin, yet they still kill each other. This is very odd, and it's a problem because it leaves me no challenge when I have to fight lots of bandits to fight.

I'm quite the novice when it comes to the creation kit, so I'm not sure what to do. Can anyone please help?

Did you save with the your mod installed? Some changes stay even after a mod is removed. I think that once the npcs respawn after 31(?) days they'll be back to normal.