I was in St. Petersburg and Moscow last year and spent a month in Kyiv. I've been to Czech and Slovak republics the year before. Albania, Croatia, Slovenia, Montenegro, Bosnia, North Macedonia, Serbia, Belarus. Next on the list is Georgia, Armenia, Romania, and Bulgaria. I've been wanting to go to Kazakhstan and Uzbekistan too.
The percentage of the population that ended up spending time in a work camp was about 2.5% and the average sentence was 2-3 years. It's terrible what happened, there's no two ways about it, but the transition from feudalism to capitalism was pretty horrendous beyond words.
Honestly, the history of the region is so complex that it's incredibly challenging to know where to begin.
As to the ideology, I agree with you about fundamentalism leading to death and destruction. Marxism, however, contains the premise, that one particular class must be liquidated. Also, it requires people to behave in a very particular and unnatural way leading easily to a conclusion that these should be eliminated too. So maybe it lends itself better as a tool for madmen raising to power, than some others. Maybe it’s not that people trying to implement Marxism have ended up in chaos but that people seeking absolute power have tended to use it as an ideological cover and to rally a support?