How would you find and return a player with only part of their name?

For example, typing /kill su would kill the user Superburke.

The other questions don't deal with whether there are multiple matches. Usually if you want to run a command, say "/ban User", then with their answers it'd ban any user in the game that had a name such as User, Username, User1, User2, etc.

You need to detect if it's ambiguous.

Here's a nifty little helper function I wrote a while ago, ported to lua:

function partialMatchUser(usern)
	local exact = game.Players:FindFirstChild(usern)
	if (exact ~= nil) then --If you put in a exact name, it should get the exact player every time
		return exact
	end
	--Do matching, no exact match
	local users = game.Players:GetChildren()
	local matches = {}
	for i = 1, #users do
		local unmLower = users[i].Name:lower()
		if (users[i].ClassName == "Player") and (string.sub(unmLower, 1, usern:lower()) == unmLower) then
			matches.insert(matches, users[i])
		end
	end
	if #matches == 1 then
		return matches[1]
	end
	--Returns nil if no matches or ambiguous
end

local command = "/kill flub"

local names = {"flooby", "flubby", "bubs"}

command:lower()

for _, s in pairs(names) do
	local name = s:lower()
	print(name:sub(1, #command - #("/kill ")) == command:sub(1 + #("/kill "), #command))
end

I'm too lazy to explain how that works fully but it's pretty self explanatory and I'm pretty sure it's what you're looking for. If the characters after the "/kill " command match the characters of the string being checked in the names table (starting from the first character of the string) true will be outputted.

If you wanted to make that more "roblox" friendly, you'd do something like this,

local command = "/kill flub"

command:lower()

local found = {}
for _, player in pairs(game.Players:GetPlayers()) do
	local name = player.Name:lower()
	if name:sub(1, #command - #("/kill ")) == command:sub(1 + #("/kill "), #command) then
		table.insert(found, player)
	end
end

if #found == 1 then
	pcall(function() --Because I'm lazy
		found[1].Character:BreakJoints()
	end)
else
	print(#found == 0 and "No one found" or "Ambiguous")
end