I guess it's not really commonly known outside the left because centrists don't really like to acknowledge our own monsters, but under Nixon's administration the operation of much of the Vietnam war, and its expansions in Cambodia and Laos, were centralized under the National Security Advisor's office and Kissinger personally oversaw and micromanaged much of the bombing campaigns that slaughtered countless civilians:
He also promoted tacit American support for the Khmer Rouge, after enabling their rise to power in the first place:
These are just a couple of examples of why he was so horrible. The number of different atrocities he was personally involved in is shocking, but it is well documented. Oh, and he committed treason, as Fastbreak notes, when he helped Nixon sabotage the 1968 peace talks in hope of landing a job in Nixon's administration. What is really wild is that he was at the same time serving as an advisor to Humphrey's campaign and Humphrey was going to give him the same fucking job Nixon gave him. I guess you can infer that he helped Nixon because he thought Nixon was more likely to win with or without his help, but I think it's pretty reasonable to infer that he supported Nixon because he knew he'd have freer reign to do whatever he wanted under Nixon than Humphrey.
A lot of people who don't even love his foreign policy have this view (probably because it's what they grew up hearing) that he may have done or condoned some bad shit, but did so because he thought it served a greater purpose. But that's a myth. He never really had any concrete foreign policy views that he was trying to advance, he just thought he should have power. He was an all time, duplicitous piece of shit who was more than happy to say whatever he needed to say to obtain more power and betrayed tons of his own friends to protect his own ass. He just liked playing the game and over a million people are dead as a result, not to mention the physical harm to tens of thousands of American veterans who survived Vietnam and the enormous psychological effects that the Vietnam War had and still has to this day on the American people (especially the American right).