This addon is great if you want to make a world where you gain certain things from killing mobs or other players, such as a Kitpvp world where you can unlock kits by killing players, or a Mob Arena world where you can buy items using money from killing mobs.
The Kill Counter addon has two different versions: Player kills and Mob kills. The Player kills addon WILL NOT WORK if you have another addon with a player entity script with a priority higher than the Kill Counter addon. If the Kill Counter addon has a higher priority, then the other addon will not work, so if you're having problems with a world with multiple packs, that might be why. To make the addon work in your world, first type: /scoreboard objectives add Kills dummy [insert whatever display name you want]. Next, put down a repeating command block, set to always active, with the command: /function kill. After you've done this, the kills will be counted!
Installation
You must turn on ALL THREE experimental gameplay options for the addon to work.