If you’re talking about really low probability deaths then you need to consider really unusual cases.

Fires gun at fleeing suspect, bullet hits _ -> deaths. Now, how does _ end up with 10 million deaths?

Well, the US Air Force for example has accidently dropped nuclear warheads on several occasions. They are designed to be really tough, but there is probably a non-zero chance a bullet could end up setting a defective one off.

Of course reality can be far stranger than fiction so I am unlikely to think of the most likely risk.

PS: The odds of terrorists being able to kill 10 million people tomorrow is so low it's really not worth considering.