Tool keeps breaking?

Your debounce pattern is wrong.

Notice the early returns on line 8/9.

You don't set debounce to false, so forever after debounce will now be true and the taser will be disabled.

Move debounce = true to happen after your early returns.

EDIT: I caught another bug.

You can't use ~= nil to see if an object exists. Use :FindFirstChild instead.

Tab your code correctly! It makes it much easier to read.

• Increase a tab when you start a new block
  • then, do, repeat, etc.
  • (blocks are groups of statements)
• Decrease a tab when you end that block
  • end, until, etc

debounce = false

script.Parent.Touched:connect(function(part)
    if not debounce then
        local human = part.Parent:findFirstChild("Humanoid")
        local a = part.Parent:GetChildren()
        if not human or human.Health <= 0 then
            return
        end
        debounce = true
        if part.Parent.Head:FindFirstChild("face") then
            part.Parent.Head.face:Remove()
        end
        local d = Instance.new("Decal",part.Parent.Head)
        d.Texture = ("http://www.roblox.com/asset/?id=145854465")
        d.Face = ("Front")
        local s = Instance.new("Sparkles",part.Parent.Torso)
        for i = 1, 5 do
            for _, v in pairs(a) do
                if v:IsA("Part")then
                    v.BrickColor = BrickColor.new("Teal")
                end
            end
            wait(0.1)
            for _, v in pairs(a) do
                if v:IsA("Part")then
                    v.BrickColor = BrickColor.new("Really black")
                end
            end
            wait(0.1)
        end
        human.Health = 0
        s:Remove()
        debounce = false
    end
end)

Personally, I would make the kill part of it a seperate function, such as

function kill(p)
if p.Parent.Humanoid ~= nil then
if p.Parent.Humanoid.Health ~= 0 then
if p.Parent.Head.Decal ~= nil then--might be face instead of decal, I generally don't deeal with more then the basic structure of players
p.Parent.Head.Decal:Destroy()
end
local d = Instance.new("Decal",part.Parent.Head)
d.Texture = ("http://www.roblox.com/asset/?id=145854465")
d.Face = ("Front")
char = p.Parent
local s = Instance.new("Sparkles", char.Torso)
xTime = 0
while xTime <= 5 do
for _, obj in pairs(char:GetChildren()) do
if v:IsA("Part")then
        v.Anchored = true --delete if you want, I just think a paralyzed pose would be cool
                v.BrickColor = BrickColor.new("Teal")
end
end
wait(0.1)
for _, obj in pairs(char:GetChildren()) do
if v:IsA("Part")then
        v.Anchored = true --delete if you want, I just think a paralyzed pose would be cool
                v.BrickColor = BrickColor.new("Really Black")
end
end
wait(0.1)
xTime = xTime+1
end
char.Humanoid.Health = 0
end
end
end

I just find it easier to edit that way, and you still get everything you want. also, to run it,

working = false
--declare function kill() here!
script.Parent.Touched:connect(function(part)
if working == false then
working = true
kill(part)
working = false
end

And remember to make sure the kill function is announced before it is called. I just didn't like the format of yours, and this should fix it up. That and the humanoid health is checked before hand so nothing breaks if stuff is already being destroyed in fire.