There are two ways to name the wrong in mass murder: as crimes against individuals, or as an attempt to destroy a whole group. A pit of thousands of bodies holds both at once: "individuals each, together a group." Which one the law protects shapes what counts as the crime.
Some killings are so grave that a nation's borders should not shield the people who order them. Once these crimes had legal definitions, a court beyond the border could judge and punish the officials responsible, whatever they said in their own defence. This made the individual, not only the state, the subject of international law.
Memory, crime and guilt do not end with the people who lived through them. They leave marks on children and grandchildren, often as family secrets that go unspoken. The past keeps shaping the present, so forgetting is not an option.