Fire for moral reason. Lying, cheating, stealing, laziness etc.
If people do those things once, chances are good they will keep doing them. Simply because they were caught once doesn't mean they will never do it again, it means they will try harder to not get caught the same way.
Another one is inability to learn or change. As someone else mentioned, if they make the same mistake and are told about it multiple times and can't stop, that means something. If they show zero ability to get better at anything. I mean, if you work with Excel for 2 years and still don't know how to do some visual design or a use a basic formula for some magic, that's an issue.
For me personally, if I have to use a tool regularly, I've got the user manual downloaded, I'm looking for hints and tricks, and I'm looking to automate repetitive tasks, and I'm occasionally testing alternate tools. I'm watching Youtube videos, or taking online classes. People who are stuck in their ways cannot drive a business forward, they can only plant it securely in place and hold up status quo.
A bad decision, sure. If your tech opens a game server and a bunch of router ports and starts serving torrents for a hacked game off your company server, that's pretty dumb. This shows complete carelessness, not just a lack of some knowledge.
If they can't be bothered to back something up before making a huge change. I understand many things in tech are complicated, and some things potentially have dozens of possible solutions, but the very "basic" stuff like do a backup before a major change, is base level.
On a personal note, I would probably fire butt kissers. These people drive me nuts. They not only butt kiss, but they will throw others under the bus for their own gain, to get an edge with the boss. A little office gossip, a little "just look what she did!"
I'd fire the boss/manager for falling for butt kissing.