You are correct.
Here's the difference:
The event is a crime, it's a personal war against society. It's graffiti with violence. As soon as it is over, remove the evidence, else it becomes encouragement.
However if the evidence is an act of violence by a large group such as another country, then it is history for all and should be preserved.
9/11/2001 a case in point. Violence by a small group got lucky. Erase the damage.
It is part the longest running , biggest war on Earth. Islam against everybody else.