Introduction: 2-3 sentences where you introduce what crimes against humanity are, and what your essay will be on.
Crimes against humanity facts: in this paragraph, you can talk about the origin of crimes against humanity, where they are dealt with, why they are dealt with.

You can use any relevant facts.
Bosnia-Herzegovina: here you can talk about what happened in Bosnia with respect to crimes against humanity.
Genocide in Rwanda 1994: in this paragraph, you can talk about what happened in Rwanda in respect of crimes against humanity.
Other crimes against humanity incidents: here you can research other incidents that have happened in the world that are also a crime against humanity and use it to argue your topic.
Conclusion: do not state any new information here. Simply state again whether you believe that crimes against humanity can be justified or not, and a summarised version of why. 3-4 sentences.