The females in this manga are really stupid. The only way her actions can be justified is if she completely discounts Ares as a power resource in a fight for control. So she either has little to no awareness of Ares or Ares' relationship with Kai (which is a scheming bitch 101 fail), or she thinks the uncle's control of the court is enough (which ignores the fact that Kai is the official head of the court and he has Ares), or she has some armed help behind her to neutralize Ares (which, through this action, will alert him to investigate).
Even if she murders the maid this will get noticed... unless the maid becomes stupid too and doesn't say anything... I guess that would make the 100% stupid female characters official.
If I had to guess... I'll go for scheming bitch 101 fail.