Many consider Russian President Putin to be a war criminal. Putin has supported the brutal Assad regime in Syria which has killed hundreds of thousands in the Syrian civil war. In fact, Assad even used chemical weapons on his own people; something that Putin refused to condemn; and continue to support Assad. Furthermore, Russia has invaded Ukraine without provocation, so Russia could annex Crimea. In short, Putin has behaved a lot like a war criminal.
Vladimir Putin is a war criminal. His latest act was dropping containers of toxic gas on a Syrian town. This hurt and killed many civilians. He has made not attempt to avoid civilian casualties,. Deliberately targeting civilians is considered a war crime. He has also annexed territory such as Crimea.
If Putin supported this action it would make him a war criminal. This does not appear to be an accident, and the Russian military appears to know that the Syrian town is filled with ordinary citizens, not just members of the military. These actions are considered war crimes under international law.
Putin has made his intentions to expand Russian borders as blatant as possible. The problem is that Russia's position in the world is too volatile for NATO to actually stand up to him. Everyone's just hoping that Putin dies in the next couple years to solve a problem that can't be solved with diplomacy and war.