Have Kill For Money Script, But Gives To Wrong Player?

Your code is almost right. Once you find the creator tag, it will hold the value of the person that killed us, so we need to find their leaderstats.Cash and add to it.

game.Players.PlayerAdded:connect(function(player)
    local folder = Instance.new("Folder",player)
    folder.Name = "leaderstats"
    local currency1 = Instance.new("IntValue",folder)
    currency1.Name = "Cash"

    player.CharacterAdded:connect(function(character)
        character:WaitForChild("Humanoid").Died:connect(function()
            local tag = character.Humanoid:FindFirstChild("creator")
            if tag ~= nil then
                if tag.Value ~= nil then
                    -- Here tag.Value is the player who killed us, so find their leaderstats
                    local killersLeaderstats = tag.Value:FindFirstChild("leaderstats")
                    if killersLeaderstats then
                        -- Award the cash to the player that killed us
                        killersLeaderstats.Cash.Value = killersLeaderstats.Cash.Value + 10
                    end
                end
            end
        end)
    end)
end)

Please use a code block instead of inline code.

As for the script, you are giving the player who died the coins instead of the player who killed. You should be getting the player who killed through the value of the creator IntValue inserted into the Humanoid:

game.Players.PlayerAdded:Connect(function(plr)
    local stats = Instance.new("Model")
    stats.Name = "leaderstats"
    stats.Parent = plr
    local currency1 = Instance.new("IntValue")
    currency1.Name = "Cash"
    currency1.Parent = stats
    plr.CharacterAdded:Connect(function(char)
        char:WaitForChild("Humanoid").Died:Connect(function()
            local tag = char.Humanoid:FindFirstChild("creator")
            if tag and tag.Value and then
                plrWhoKilled = tag.Value
                if plrWhoKilled:FindFirstChild("leaderstats") and plrWhoKilled.leaderstats:FindFirstChild("Cash") then
                    plrWhoKilled.leaderstats.Cash.Value = plrWhoKilled.leaderstats.Cash.Value + 10
                end
            end
        end)
    end)    
end)

Keep in mind this will only work with weapons that insert this creator value into the humanoid on kill.