If the reactors are on, destroying any of the infrasture around it can be devastating. The reactors need external power to operate. If external power disappears, the reactors now need emergency power from diesel gen sets. If those fail due to any reason, the reactors will probably melt down - unless they have already been off for quite a while (~weeks would be necessary). If the reactors melt down, the VVER has both a core catcher and a containment. That's not bad and much better situation than Chernobyl. But other things may go wrong such as a pressure failure and potentially related control rod ejections that could cause the temperatures to rise and pressures to become higher than what the containment can handle.

The surrounding war environment, the stressed operators, etc mean that any of these failures are now more likely. It is reaonsable that external power will not be available, it is possible the gen sets will fail, it is possible the operators will do something stupid.

To put it simply, warfare is not a design basis accident for current light water reactors like VVER-800.