Leaderstats kills not working?

I see. Player.CharacterAdded won't fire when a player joins or the Players.PlayerAdded event. To fix this, you create a Script inside StarterCharacterScripts.

Why put in StarterCharacterScripts? It's because everything inside StarterCharacterScripts will be moved to the player's character once it loaded/respawned. Not only it will be moved, but making a Script a descendant of Workspace (Player.Character is a descendant of Workspace too) will automatically start running the script.

After creating the Script, copy this code:

local Players = game:GetService("Players")

local Character = script.Parent
local Humanoid = Character:FindFirstChildOfClass("Humanoid")
local Player = Players:GetPlayerFromCharacter(Character)

local Stats: Folder = Player:FindFirstChild("leaderstats")
local Kills: IntValue = Stats:FindFirstChild("Kills")
local Deaths: IntValue = Stats:FindFirstChild("Deaths")

Humanoid.Died:connect(function()
    for _, Child in pairs(Humanoid:GetChildren()) do
        local Killer = Child.Value

        if Child:IsA("ObjectValue") and Killer and Killer:IsA("Player") then
            local OtherStats = Killer:FindFirstChild("leaderstats")
            local OtherKills = OtherStats:FindFirstChild("Kills")

            if OtherStats and OtherStats:IsA("Folder") and OtherKills and OtherKills:IsA("IntValue") then
                Deaths.Value += 1
                OtherKills.Value += 1
            end
        end
    end
end)

Make another Script inside ServerScriptService and this is where we make the leaderstats.

local Players = game:GetService("Players")

Players.PlayerAdded:Connect(function(Player)
    local Stats = Instance.new("Folder", Player)
    Stats.Name = "leaderstats"

    local Kills = Instance.new("IntValue", Stats)
    Kills.Name = "Kills"

    local Deaths = Instance.new("IntValue", Stats)
    Deaths.Name = "Deaths"
end)

After doing all that, you may now delete your old leaderstats script.