The fascinating thing with Kissinger is that he's still alive, still wields political influence, and has been active for eighty years and was wielding his influence in essentially all major conflicts during that time. He just published another book and just celebrated his 100th birthday.
He started out as a German Jewish refugee. Fought in WW II and became an American, ended up being part of the post WW II effort to administer Germany, and then started his career in political science and international politics.
The nineteen sixties and seventies were a complicated era. The war in Vietnam was an ugly affair, and Kissinger's involvement with negotiating the peace deal is what earned him a controversial Nobel peace price. It was also the peak of the cold war and Kissinger was basically coordinating policy in a number of proxy wars across the world. Iran/Iraq, Chile, Vietnam, etc. and projecting US power everywhere he could.
Calling him a war criminal is a bit rich. But undeniably some of the decision making at the time backfired in a big way and Kissinger was at the core of a lot of that. Both during the Nixon era and long after when he advised administrations all the way until now.
Particularly the decision to prop up a lot of corrupt despots in order to prevent the Russians from gaining influence has caused a lot of long lasting conflicts that continue to play out today. You could make the point that the US ultimately won the cold war. Albeit at the cost of triggering a lot of other ugly conflicts.
The Iranian revolution is probably one of his bigger failings. Triggered directly by him backing the wrong horse in Iran, which ultimately lead to the Iranian revolution, the US providing military aid to Iraq, the Iran/Iraq war, the Iran contra affair (where the US provided weapons to Iran) the Russian/Afhanistan conflict where they US also provided lots of military aid, both Gulf wars, more recently the Afghanistan and Syrian conflicts. And there's of course the decades long support for the brutal regime in Saudi Arabia, the US involvement in various revolutions across Africa and South America. There's a pattern there of the US backing the worst regimes in order to divide and conquer and gain/expand its influence and power. That's what big countries do.
Was Kissinger involved with and personally responsible for all of that? Probably not but he was definitely advising a lot of politicians and this soft power via military aid, diplomacy, meddling, propping up despots, etc. definitely is his style. But he was only in office for a few years and during that time his big focus was ending an ugly war in Vietnam that he did not start. There were hard line politicians before and after that. He certainly has been on the sideline advising many of them for decades. Especially the hard line policies under the Reagan and both Bush presidencies have his finger prints all over them and he also consulted Democratic presidents like Carter, Clinton, Obama, and Biden. But does that make him a criminal? If so, there are a lot of other war criminals. And they are enabled by voters who keep voting for this sort of thing.