Why?
Because she is a Goddess with a visible presence to the people in Yliakum, thus entitled to any worshipers that will follow her.
Why?
Because that is how the gods work.
Why?
Sorry, you don't get the 'why' of that.

So, it is ok to be a murderous, cutthroat bastard if I say I worship Dakkru, and can not be punished by the law?
No. You will be punished for breaking any Octarchal law, including murder (any killing of a citizen outside of a duel). Just as you would be punished for murdering someone 'in the name of Talad'. There are killers in all four religions, and none will be tolerated by law.