For me, the general order of precedence, from most influential to least, seems to be:
1.) Introduction of a MAJOR character (and by this I mean, Superman, Spider-Man, Batman, Wonder Woman, Flash, Green Lantern--that "A list" level)
2.) Death of a MAJOR character (although some might bump this above the introduction--I mean which is a more important issue, the introduction of one of the countless Flashes, or the death of one. Then again... they don't tend to STAY dead and that devalues their death stories).
3.) Major comics "first", as in your example of the first genuine hero murder in a superhero comic book (as opposed to death in the line of duty, which is a different thing entirely, even if a supervillain was responsible in that case too).
4.) Introduction of a moderately important character
5.) Death of a moderately important character (although this may leap ahead of the introduction if the death is especially shocking).

On the other side of the grave, Pep Comics #17 contained probably the first actual, no-lie murder of a superhero in comics when the Comet was killed and the Hangman was introduced. Those Archie comics were hardcore.

The Fury of Firestorm #17 in turn introduced Firehawk.

Steve Gerber's great twist on the Superman archetype.

and

MAN-THING v1 # 17 was a powerful story by Steve Gerber about book burning.

~P~
PTOR

I've got a huge sentimental spot for Love & Rockets #17. It was my first issue of the series, and the two stand alone stories in the issue were a perfect introduction to the Hernandez Brothers and to the punk rock scene. A massively influential comic on my teenage years.