If something terrible happens, one can nearly always say that there have been even more terrible things. Only nearly always, because there's a category of events that are so close to the extreme limit of the horrors human beings have inflicted on one another that it's hard to know whether the other examples one might point to are more terrible or merely just as terrible. It's therefore not a good idea to engage in this exercise for the sole purpose of telling the world that there are things more terrible than some given terrible thing. That there are is well-known, and your saying so does rather suggest a dismissive, diminishing attitude to what ought not to be treated in this way.
Doris Lessing shows herself impervious to such considerations - saying of 9/11 that it 'wasn't that terrible'. It was terrible, but not 'that terrible':
Do you know what people forget? That the IRA attacked with bombs against our Government.She could, of course, have chosen Rwanda or the Cambodian genocide, and then the reaction to 9/11 would really look like making a mountain out of a molehill.
Doris Lessing is a writer of distinction and achievement. But we already know that distinction and achievement don't necessarily save you from saying shameful things.