What’s stopping the downloaded script from wiping my home directory?

What’s stopping any Makefile, build script, or executable from running rm -rf ~? The correct answer is “nothing”. PPAs are similarly open, things are a little safer if you only use your distro’s default package sources, but it’s always possible that a program will want to be able to delete something in your home directory, so it always has permission.

Containerized apps are the only way around this, where they get their own home directory.

Who maintains uutil? Presumably it’s not GNU?

Always follow best practices, make sure you cleanup by deleting the hello world bash file!

No, you don’t. Containers are the endgame of a bunch of dumb people saying “I don’t like apt, so I’m going to make my own and it’ll be better in my own distro”, and now we have a hundred incompatible alternatives that are worse than apt, and no one knows how to deploy for all of them, so they give up and make a container.