Yes, just running a random script from the internet is a very bad idea. You should also not copy and paste the command from above, since I'm only a random lemmy user. Nevertheless, if you trust k3s, and they promote this command on the official website (make sure it's the official one) you can use it. As you want to install k3s, I'm going to assume you trust k3s.
If you want to review the script, go for it. And you should, I agree. I for myself reviewed (or at least looked over it) when I used the script for myself.
For the uninstallment: just follow the instructions on the official website and run /usr/local/bin/k3s-uninstall.sh source
Wieso verwendest du Tusky und Mastodon?