Should destructors be threadsafe?

The destructor should not be called when the object is in use. If you're dealing with such a situation, it needs a fundamental fix. However, the destructor might want to alter some other thing (which is unrelated to the class being destructed) and it might need a critical section (e.g. like decrementing a global counter).

I think you have a more fundamental problem. It shouldn't be legal to destroy your object on one thread while another thread is still calling member functions. This in itself is wrong.

Even if you successfully guard your destructor with critical sections, what happens when the other thread starts executing the remainder of the function? It will be doing so on a deleted object which (depending on it's allocation location) will be garbage memory or simple an invalid object.

You need to alter your code to ensure the object is not destructed while still in use.

If you're accessing global variables you might need thread safety, yes

eg. My "Window" class adds itself to the list "knownWindows" in the constructor and removes itself in the destructor. "knownWindows" needs to be threadsafe so they both lock a mutex while they do it.

On the other hand, if your destructor only accesses members of the object being destroyed, you have a design issue.