I can't speak to a US perspective but in the UK and some EU countries I've experienced, firing someone is incredibly simple.
In fact every difficulty I've seen is simply that someone didn't follow the clearly defined procedure.
It's literally written out for you. You don't have to think or care how you feel, just follow the process and you're done. If the process says someone should stay then you got something wrong. Simple as that.
To be clear, it's simple for a *company* to fire someone. It often can be a pain in the ass for a *manager* to do it. For instance, a process like this:
1) Need to set up a clear paper trail over a period of time. For instance, a track record of being marked as an underperformed in their reviews with concrete complaints. In places that require this to be tied to the review, and f they only have annual reviews, this can take a LONG time.
2) Bring HR into the process, where they'll do the equivalent of "did you turn it off and on again?" for quite some time
3) If they let you, set up a PIP, which itself will take several weeks
4) Finally the person is let go