What do the Twin Towers and Anne Frank have in common? They're both just piles of ash now a days.
What makes you feel that it's okay to joke about people being killed?
Hey, who cares about Anne Frank and those people in the Towers. They are nothing.
People dying in the Twin Towers was a traumatic event and it minimizes their memory when you joke about them. Take others difficult experiences seriously or no one will take you seriously.