"for the supremacy of the state's interests over any alliances or the security and integrity of other states"
I'm pretty sure that's the definition of patriotism instead of realism. Being a realist means that you assess the situation and how your environment (in the MC's case, the world since he's a ruler) works. Asking questions like, "Is it possible to avoid war? Why is it possible or not possible? What's the impact? How would it affect me or my people? What options do I have? Which option has the most benefit? Which option is the most risky? Which option is practical to be done at this stage? How would that option reflect in the long run?" By asking these questions and ultimately coming to several routes, then one typically realizes that often times, the one yielding the highest probability for bringing benefits to you or your people don't come free and that sometimes lives must be taken out. But even still, one understand that it's "every man for himself" in the end and as long as it's not overboard (e.g. conducting massacre of people including those who are completely unrelated by any reasonable doubts), that's what you call a realist. It's the ability to objectively analyze the situation, and take action appropriately while having your priority sorted with common sense.