It was poor reactor design combined with downright stupid actions by management. But a nuclear plant should be somewhat stupid-proof.

For whatever it's worth, Russia continues to operate several RBMK reactors today, and haven't blown any of them up since Chernobyl in the 80s. This is probably attributable to some combination of the retrofits they made after Chernobyl, generally treating the matter more seriously, and maybe luck.

https://en.wikipedia.org/wiki/RBMK#List_of_RBMK_reactors

Retrofits have not generally occurred. The person most personally responsible was given control of another one.

The retrofits did occur. The second sentence of your comment does not relate to the first; 'retrofit' does not mean "change in management"

We know at least some retrofits were done.

That's taking a blameless post-mortem approach which is a valid tool, but not the entire answer.

The reality is to cause the issue, the operators had to drive the reactor well into a dangerous and hard to control regime which it would not get into under any normal operation circumstance.

So while yes, it shouldn't have been physically possible to do it, even with that design it took substantial, deliberate malfeasance to get that result (you can also only get that result with that design - a meltdown is not normally an explosion).

From what I’ve read some of the safety systems did not give real-time feedback and the critical error occurred because the operators didn’t know they were in trouble due to lack of real-time feedback. I’m not an engineer and know nothing of these things but the test was not unauthorized and it wasn’t an experiment. As far as I read about the accident.

Many causes can be attributed to the disaster.