Kissinger is a realist and knows the longer this war goes on te more land Ukraine is likely to lose, leaving them a rump state minus a coastline on the Black Sea. And if NATO were to intervene, the odds of a tactical nuke being employed is very high. That leaves a negotiated settlement as the best option at this point.
Kissinger is a fossil and a tool. His brand of realpolitik is precisely why the world is so effed up.