The Gautama Dharma Sutras on sacred-texts.com apparently says there that it's not a sin to kill a prostitute:
- For (killing) a Vaisya (the same penance must be performed) during three years; and he shall give one hundred cows and one bull.
- (The penance for killing) a cow is the same as for (the murder of) a Vaisya.
(For killing a woman who subsists) by harlotry, nothing at all.
Is there a commentary for this verse? sacred-texts does not mention any commentary for this verse. Haradatta is one commentator for the Gautama Dharma Sutras. He comments on other verses but not this one according to sacred-texts.