Given that I've stated of Ron Marz's decision to kill off his character Alex--
"I support his right to come up with a story in which a supporting cast-member is horribly killed simply to advance a particular plotline"
--but also stated that I didn't think his story was very good-- it behooves me to mention a couple of times when support-cast members were killed to reasonably good effect.
I've mentioned that I didn't like Gwen Stacy's death in AMAZING SPIDER-MAN #121. That's not because I thought the character was indispensable to my enjoyment of the magazine, but because I felt that that the writers during that period-- not just Gerry Conway, but also Stan Lee in his last 10-20 Spidey-scripts-- made her death predictable by virtue of failing to expand on her character beyond the usual cliches. I think now that it made sense to kill her from an editorial standpoint, but still dislike Conway's hamhanded execution.
In contrast, Stan Lee himself "put a hit out" on one of the characters introduced in the early years of John Romita's tenure: Captain George Stacy, father of Gwen. Like Gwen, George dies in a senseless accident, but Lee gave him a heroic death. When a rooftop-fight between Spider-Man and Doctor Octopus causes heavy bricks to plummet to the street below, Stacy perishes in shoving a small child out of the way of the debris.
Now, to some extent this is more palatable than Gwen Stacy's death because it is a heroic death. However, what I like best about the event is not just that the death was heroic, or even unexpected (certainly more so than daughter Gwen's), but that it became a new source of narrative avenues, allowing for a good amount of "my-sweetheart-hates-my-superher0-alterego" melodrama.
Another example-- perhaps more appropriate to the theme of senseless deaths-- would be Iron Man's 1960s girlfriend Janice Cord. Janice, introduced in IRON MAN #2, remained a regular support-cast member for the next 10 issues, only to meet an untimely end in IRON MAN #22, when she was caught in a crossfire between Iron Man and his enemy Titanium Man. Janice Cord hadn't been in that feature as long as Gwen Stacy had appeared in SPIDER-MAN, and thus it's quite possible that she, like Ron Marz's Alex, was intended from the start to be a sacrificial lamb. Scripter Archie Goodwin does allow her a Last Moment as she dies in Iron Man's arms, almost recognizing him as Tony Stark before she succumbs. Still, she was a pretty conventional character, having little identity apart from being Tony Stark's girlfriend, and so in a sense one might state that she most "came alive" in the intersubjective sense when she suffered the fate of a disposable support-cast member.