Marx wrote books, and he died in 1883. The split between Marxists/Trotskyists/Stalinists is well-documented, and reading Das Kapital, you'll see it takes as much of a twisted mind to go from his ideas to Stalin as it takes to get from the bible to the Westboro Baptist Church.

Kissinger was an active politician, and he's criticised not so much for his ideas as his actions. I seem to remember that quite a few people during the Charlottesville brouhaha were keen to insist that people should be judged by their actions.

There's also a difference between a study of Marx/Kissinger, and attending an event that has them as guests. A famous speaker's attendance is an honour for the host, but it also honours the speaker. I'm sure there are scores of left-leaning researchers and students who have read Kissinger's work or studied his actions while being highly critical of him.

The obvious example is that it's just as common to read Hitler's book and speeches as it is to read Marx when studying history.