Hi, Nika :)
Really the best thing you can do about this is exactly what you did do, so I wouldn't worry about it. Kat is going to see other disturbing things in her life, some real and some not. It's all part of growing up.
There have been disturbing things written long before John Patmos' "Revelations" and long after. Whether Emmerich's movie attains the status of "Great Art" I guess we won't know until it comes out. Will Smith put on a great performance in "Independence Day" but that movie kind of "blended in", so we'll see.
The truth is, Emmerich has First Amendment rights, just like you or I. Pets die. Stevie Nyx's uncle John died. Presidents die.
I'm trying to remember some of the songs I've heard at open mics; Patty Finney's song "One Dead" about when she was washing dishes and saw a traffic accident though the window and then used the vacuum cleaner as the TV told her about it was pretty disturbing. Rusty Wier's "Black Magic Gun" was a great song about evil. There's my own "Don't Drive 105", so I followed with "The Dead in Heaven" (kind of a counterpoint). So I guess I'm in the same boat as Emmerich.
You're teaching Kat healthy skepticism, which is good.
I think you'd make a fine Mom.
:D