Board Thread:General Discussion/@comment-76.30.106.111-20130110233823/@comment-3157150-20130207034456

The fact that Yrimir killed it is irrelevant. The original purpose of the Dungeon Guardian was to stop players from entering somewhere they weren't "cut out" to enter, the judgement being made on them beating Skeletron.

tl;dr: The fact that the Dungeon Guardian is "killable" is effectively a glitch in the game. Simplest way to say it.

Also, I'm closing the thread as the answer has been given several times.