Tachahashi Matavreka shortly answers the questions:
Selfishness, pride, greed, lust for wealth, power and pleasures, are the main reasons that either a person or a society end up badly. Humans are more prone to follow the ‘downward path’ than put effort to reach ‘higher’.
If a Polity isn’t organised with a system that teaches and habituates its civilians towards Good, Coherence, Prudence, Virtue, etc. then self-destruct, sooner or later, with better or worse terms, WILL eventually come at some point.
Now, as for an individual, no, nothing is ‘inevitable’. Every one of us is responsible for himself. If one understands his mistake(s), then it is upon him to correct those! If not, he chooses his Fate alright. And no one else is to blame.
The only ‘excuse’ that one might have is been brought up in a Polity, or an environment (Family has huge responsibilities also) that nothing like Logic, Prudence, Virtue, etc. was taught to him. Then it is apparently extremely difficult to convince such a person that he is wrong in the first place.
And this is a tragedy we observe all over the planet.