Because people dying with Illithid larvae inside their heads don't die but become enslaved and live on millenia? It's not murder if the person continues to live with a parasite, right? So a god of murder miiiight loose its divine spark even though I assume there is more than enough actual murder going around.
My cynical self assumes that Larian's writers just asked Wizards: "What are the most tentacly sticky things unique to Faerun that we can use as villains." and their response was Aboleths and Mindflayers.