> Not so if the shot down people can also make the case that they feared for their lives

Can you cite the specific German statute or case history that says this? Or is it just what you intuitively expect to be true?

> There is a good reason why only highly trained government forces are allowed to carry a weapon let alone some teenager.

For what it's worth, I wish that were the case in the US too. But whether carrying a gun is illegal is a different question from whether using it in this specific situation makes you also guilty of the additional crime of murder.