If your base class destructor is virtual then objects will be destructed in a order(firstly derived object then base ). If your base class destructor is NOT virtual then only base class object will get deleted(because pointer is of base class "Base *myObj"). ... Therefore destructor should be pure virtual.

Should I always make destructors virtual?

Virtual keyword for destructor is necessary when you want different destructors should follow proper order while objects is being deleted through base class pointer.

Why do we need virtual destructors?

Virtual destructors in C++ are used to avoid memory leaks especially when your class contains unmanaged code, i.e., contains pointers or object handles to files, databases or other external objects. A destructor can be virtual.