A content warning can be added even after posting by editing the post. (Remember, this is not Xitter. You can edit posts here.)
And yes, please... A #ContentWarning for this level and detail of gun violence is absolutely necessary. I am not one to add CWs lightly, but it is vital here.
Also, I don't know why ISIS would attack a Moscow theatre/mall, but they are taking credit, per CNN.
HistoPol