It would be a good question to askphilosophy. The main problem with realism as I know it are semantics: who are those "nations" and what is their "interest"? Who was that superpowerful "hegemon" whose magnificence kept Europe at peace between Napoleon and the Crimean War? Where is it even ought to apply?

As /u/hohloved suggested, you might have better luck in a subreddit more focused on political theory. It also might help if you clarified your question a bit. Are you looking for alternate theories to realism within the study of international relations which allow for states to act outside their own interest, or are you looking for explanations that realism offers for states acting seemingly not in their own interest?

Within realist theory, actions taken by a state to assist another state are generally explained by the actor wishing to gain something from the resulting situation or sometimes something in return from the other state, such as the formation an alliance or other beneficial relationship. If the state's actions to assist another state are taken to create a beneficial outcome for the actor, the actor has not acted selflessly or outside its own interest. Realism also generally characterizes such relationships as temporary; since rational state actors will seek to prevent other states from increasing their security (as doing so would result in a relative decrease in the first actor's own security. This is referred to as the security dilemma— any actor increasing its own security will spur other states into attempting to increase their security to compensate).

If you're looking for alternate theories which offer explanations for states acting outside their own best interest, it would be helpful to consider theories such as constructivism, which suggests that the realist perception of the international system as anarchic and characterized by state actors acting only in pursuit of their own security is inherently flawed and that cooperation between states can allow all cooperative states to increase their security. This idea, of course, directly contradicts the aforementioned realist concept of the security dilemma.

Finally, if you're looking for actions which are incredibly difficult to explain with realist theory, /u/DeSoulis has already provided an incredible example. The very idea of a state willingly ending its own existence is, through the lens of realist theory, absurd. In realist theory, states are unified and personified rational actors which act to increase their security in order to ensure their existence, which makes explaining any action taken by a state to deliberately bring about its own end challenging to explain.