What exactly was the original spoiler, anyway? That the Dark Brotherhood quest has a final quest, and that the final quest for a faction dedicated to assassination is well suited to a soundtrack from a game about assassination? Shocking. If someone was complaining about /that/ being a spoiler, doing some actual spoiling may be a bit mean, but it is hilarious.
On the other hand, if you gave details about said final mission in an unmarked spoiler, it goes beyond being "a bit mean" and into dickish territory. Note that I would still consider it marked if it was in a thread that was marked for spoilers in the title, whether you personally marked your post or not.