There are two main ways in which genocide is important to the study of philosophy.
First, there is what is called the "problem of evil." This is the question of whether the existence of evil calls into question the existence of God -- if evil exists can an all powerful and good God also exist? Genocide is perhaps the ultimate evil that people do and therefore impacts on this issue.
Second, there is the problem of what philosophy can or cannot do to end genocide. There have been philosophical approaches that have tried to combat genocide, but there have also been philosophical schools of thought whose teachings have helped lead to genocide. Therefore, it is important to philosophy to ask what, if anything, philosophy as a discipline can do to end genocide.