How to use datastores that saves the players total leaderstat value?

From what you asked I think you are trying to say that you want them to start from 0 each time they join but still have their overall kills for each time they have played be saved somewhere. If you are just looking for a general datastore thats fine as well this will fith both needs.

Create a folder in ReplicatedStorage called PlayerData, update whatever pvp scripts you are using to update the Kills and Deaths inside that folder alongside your current leaderstats via the hierarchy of:

game.Players = game.Players.LocalPlayer
game:GetService("ReplicatedStorage").PlayerData:WaitForChild(player.name).Stats

You can then have a place in game or on a gui to display their total career stats by pulling the values from the folder referenced above. The main script below simply goes in a script in the ServerScriptService.

I added a cash line if you have a money system you would like to save as well but you can simply remove it if it is not needed as well as copying those lines for more stats you wish to create.

Don't forget to change the key on the first line as well.

local DS = game:GetService("DataStoreService"):GetDataStore("PUTRANDOMKEYHERE")
local PlayerData = game:GetService("ReplicatedStorage"):WaitForChild("PlayerData")

game.Players.PlayerAdded:Connect(function(player)
	local PlayerFolder = Instance.new("Folder", PlayerData)
	PlayerFolder.Name = player.name
	
	local Stats = Instance.new("Folder", PlayerFolder)
	Stats.Name = "Stats"
	
	local Kills = Instance.new("IntValue", Stats)
	Kills.Name = "Kills"
	
	local Deaths = Instance.new("IntValue", Stats)
	Deaths.Name = "Deaths"
	
	local Cash = Instance.new("IntValue", Stats)
	Cash.Name = "Cash"
	
	local SavedData = DS:GetAsync(player.userId)
	
	if SavedData then --// Load Saved Data
		for i,v in pairs(SavedData) do
			if 1 % 2 == 0 then
				PlayerData[player.Name].Stats:FindFirstChild(SavedData[i - 1]).Value = v
			end
		end
	end
end)

local function SaveValues(player) --// Save Data Function
	local Data = {}
	for i,v in pairs(PlayerData[player.name].Stats:GetChildren()) do
		Data[#Data + 1] = v.Name
		Data[#Data + 1] = v.Value
	end
	DS:SetAsync(player.userId, Data)
end

game:BindToClose(function() --// Save Data on server close
	for i, player in pairs(game.Players:GetPlayers()) do
		SaveValues(player)
	end
end)

game.Players.PlayerRemoving:connect(function(player) --// Save Data on player leaving
	SaveValues(player)
end)