No, I get the feeling that killing Millie won't make the killings any more or less frequent, just take one person out of rotation. If that's the case, then either we kill them when we identify all six because we don't have the entire ritual, or we figure out the ritual before we identify them all and perform it once we have all six confirmed. Doing it this way will save as many lives as possible if I'm correct in the frequencies of the stalkers killing.