alleged-adjective
1. declared or stated to be as described; asserted: The alleged murderer could not be located for questioning. 
2. doubtful; suspect; supposed: The alleged cure-all produced no results when it was tested by reputable doctors. 
allege–verb (used with object), -leged, -leging.
1. to assert without proof.
2. to declare with positiveness; affirm; assert: to allege a fact. 
3. to declare before a court or elsewhere, as if under oath.
4. to plead in support of; offer as a reason or excuse.
5. Archaic. to cite or quote in confirmation.

At first glance it seems appropriate. Lamentable, dreadful, fatal… sure! Let’s use it!

But “tragedy” implies that it was out of the hands of Man, outside of evil intent.  A flood, a fire, a hurricane, a tornado can result in tragedy.  An expedition on a glacier that suffers a landslide, loses most of their food, and starves to death trying to escape back hundreds of miles to civilization, is a tragedy.  A military expedition that encounters dreadful weather, limiting air support, and suffering huge casualties as a result, can even be considered a tragedy.  But in every case there is an intervening event outside the motivations and purposes of Man.  A force of nature or an accidental choice – a mistake – that each leads to the calamity.  The Titanic iceberg coupled with the hubris of “full steam ahead”.  Where was Hassan’s “mistake”?  Did he think he carried a toy gun that would pop-pop and merely scare people?

No, this was a slaughter, a premeditated mass murder event.  I will vomit before I call it a “tragedy”.

Extremely unpleasant to the senses or feelings: atrocious, foul, horrid, nasty, nauseating, offensive, repellent, repulsive, revolting, sickening, ugly, unwholesome, vile.
So objectionable as to elicit despisal or deserve condemnation: abhorrent, abominable, antipathetic, contemptible, despicable, despisable, detestable, filthy, foul, infamous, loathsome, lousy, low, mean, nasty, nefarious, obnoxious, odious, repugnant, rotten, shabby, vile, wretched.