Why is the script killing me when I try to weld it up?

You set the player's torso to the player, which is why it kept killing them. The torso doesn't need to be parented. You also forgot to set the parent of the weld.

And I'm not sure why it even worked, because in lines 31-33, you used 'plr' which wasn't defined. If you're trying to loop through the players and do it for all of them, then substitute plr with v, as I did. You should also connect the function to the actual player added event, which I assume is what you wanted to do from 'OnPlayerAdded', as right now it'll only run once.

local lplayer = game.Players.LocalPlayer

local HumT = lplayer.Character.Torso

local RepStore = game:GetService("ReplicatedStorage")


local Players = game:GetService("Players")

function OnPlayerAdded(plr)
    print("Hello!")

local part2 = RepStore.Blocky:Clone()
part2.Parent = game.Workspace



local weld = Instance.new("Weld")
weld.Parent = part2
weld.Part0 = part2
part2.Parent = HumT
weld.Part1 = HumT


weld.C0 = CFrame.new(0,0,0)
weld.C1 = CFrame.new(0,0,0)


end


for _,v in pairs(Players:GetPlayers()) do
    OnPlayerAdded(v)
end

game.Players.PlayerAdded:connect(OnPlayerAdded) -- Add this if you want it to run the function when a player is added.