The whole problem is employers abusing employees by firing without any reason, and using the threat of firing employees if they complain. That is used to force employees to accept unsafe and illegal working conditions, because if you're an employee who's choice is "report workplace problems" or "pay rent" you're kind of forced into accepting unsafe or unreasonable work places.[1]

Because it turns out that is what employers do. That's why most countries have have laws preventing employers from arbitrarily dismissing employees unless you have an actual reason.

If you have a bad employee, you can fire them. You don't need at will employment for that. Unless you mean "I don't like this employee, who is doing their job properly" in which case you don't have a bad employee, you're an asshole.

[1] If you're ever fired and your employer claims they're allowed to do so because "at will" employer, note that "at will" only covers "legal" reasons. Retaliation, bias, anti-pregnancy, anti-child, anti-religion, etc are all illegal reasons. Always ensure that everything you communicate with your employer is in writing. As @nxm is demonstrating your only safe option is to assume that your employer is your enemy.