Sibling comment correctly points out that misbehavior would follow a different termination path, but I don't actually know what it is since I've never seen a European employee successfully fired. We normally just lay off problem employees and follow the same offboarding procedure for everyone. This does present its own retroactive abuses of the PIP process.
> If you know that you have X months of pay if you behave, then why misbehave?
Ageism is real. For those expecting to retire from a company in Y years, seeing expected future income reduced to X months is catastrophic since there's no guarantee they will ever continue their career in any capacity yet expect to live beyond X months. The inhumanity comes from realizing how insignificant you were to the grand scheme of things, and how easily you are discarded and forgotten.
Only the younger crowd thinks the way you do, where there's always more time to find another job. They can afford to be rational. For the rest this will be the last job they ever have; it is an indignified and humiliating end to a career they spent decades building. Revenge is easily rationalized.
Employment is modern slavery. Few earn enough to have meaningful agency over their lives.