I know it isn't a big deal, because getting your GM to add a few domains to a Great Old One shouldn't be a problem if you've already convinced said GM to let you worship one. That being said, Hastur's two main foci, no matter the source material you're pulling from, are Madness (Insanity/Nightmare) and Death (Undeath? The original material gives him a zombie servitor).
I get putting Rune down, but it really isn't about the Yellow Sign as a symbol, its about the Yellow Sign being a conduit for madness.
Void makes sense for the Derleth affected version of Hastur (though the Pathfinder version is thankfully Derleth-light), or if you take the title The Last King literally. Carcosa's sky is full of black stars, which could mean that Carcosa is not only distant locally, but also temporally far into the future (black stars=black holes).
Evil, sure.
Chaos, sure.