This script activates on left click, However you do not see a lot of it because there is too much script. So im going to show the things of the knife [Which connect on left click] only.

-This is suppose to activate when I left click, However it doesn't at first. [This is the glitch] 1.First you have to press "f" (Then it allows you to knife) 2.Then you can knife [But you can spam it, however fast you want]

..Script stuff..

On Activation:

local tool = script.Parent 
function ToolEquipped(mouse) 
mouse.Button1Down:connect(function() 
	script.Parent.Main.Transparency = 1
	script.Parent.Main.Knife:Play()
	local Connection = nil
	local Blade = Instance.new("Part")
	Blade.BrickColor = BrickColor.new("Really black")
	Blade.Name = "Blade"
	Blade.CanCollide = false
	Blade.FormFactor = Enum.FormFactor.Custom
	Blade.Size = VEC3(0.5, 2.5, 1)
	local Mesh = Instance.new("SpecialMesh")
	Mesh.MeshId = S.KnifeMeshId
	Mesh.MeshType = Enum.MeshType.FileMesh
	Mesh.Scale = VEC3(0.7, 0.7, 0.7)
	Mesh.TextureId = S.KnifeTextureId
	Mesh.Parent = Blade
	Blade.Parent = Gun_Ignore
	local BladeWeld = Instance.new("Weld")
	BladeWeld.Part0 = Blade
	BladeWeld.Part1 = FakeLArm
	BladeWeld.C0 = CFANG(RAD(-90), 0, RAD(180))
	BladeWeld.C1 = CF(0, -1, 0.75)
	BladeWeld.Parent = Blade
	Connection = Blade.Touched:connect(function(Obj)
		if Obj then
			local HitHumanoid = FindFirstClass(Obj.Parent, "Humanoid")
			if HitHumanoid and IsEnemy(HitHumanoid) then
				local CreatorTag = Instance.new("ObjectValue")
				CreatorTag.Name = "creator"
				CreatorTag.Value = Player
				CreatorTag.Parent = HitHumanoid
				HitHumanoid:TakeDamage(HitHumanoid.MaxHealth)
				MarkHit()
			end
		end
	end)
	TweenJoint(LWeld2, CF(), CFANG(0, RAD(90), 0), Linear, 0.05)
	TweenJoint(LWeld, ArmC0[1], CF(-0.1, 0.2, -0.1) * CFANG(0, 0, RAD(-20)), Linear, 0.05)
	TweenJoint(RWeld, ArmC0[2], CFANG(RAD(-30), 0, 0), Linear, 0.1)
	TweenJoint(Grip, Grip.C0, CF(), Linear, 0.1)
	spawn(function()
		local Force = HRP.CFrame.lookVector * 8e4
		local BF = Instance.new("BodyForce")
		BF.force = Force
		BF.Parent = HRP
		delay(0.03, function()
			BF.force = -Force / 2
			wait(0.03)
			BF:Destroy()
		end)
	end)
	wait(0.05)
	RotCamera(RAD(6), 0, true, 0.1)
	delay(0.1, function()
		RotCamera(RAD(-2), 0, true, 0.05)
	end)
	TweenJoint(LWeld, ArmC0[1], CF(0.8, 1.7, 0.2) * CFANG(0, 0, RAD(-80)), Linear, 0.06)
	wait(0.2)
	Connection:disconnect()
	wait(0.2)
	TweenJoint(LWeld2, CF(), CF(), Linear, 0.15)
	TweenJoint(LWeld, ArmC0[1], S.ArmC1_UnAimed.Left, Linear, 0.15)
	TweenJoint(RWeld, ArmC0[2], S.ArmC1_UnAimed.Right, Linear, 0.15)
	Blade:Destroy()
	script.Parent.Main.Transparency = 0
	end)
end

connection:

if Key == S.KnifeKey and S.CanKnife then
		if KnifeReady and (not Knifing) and (not ThrowingGrenade) then
			if Aimed then UnAimGun(true) end
			BreakReload = true
			Knifing = true
			KnifeReady = false
			tool.Equipped:connect(ToolEquipped) 
			BreakReload = false
			Knifing = false
			delay(S.KnifeCooldown, function()
				KnifeReady = true
			end)
		end
	end

Connection from module script:

CanKnife = true;
	KnifeMeshId = "http://www.roblox.com/asset/?id=121944778"; 
	KnifeTextureId = "http://www.roblox.com/asset/?id=121944805"; 
	KnifeCooldown = 0.5;

Key Connection from Module:

KnifeKey = "f";
Anonymous