He specifically calls it a "town of prosttutes" (that's a direct quote with a slight word change because of forum censoring) and says the townspeople were killed for moral deficiencies, to set an example...how much more proof do you need?
Short of beating the player over the head with a dead body, he can't really make it more explicit. Legion finds that kind of stuff reprehensible, they executed everyone, end of line. Some people who deserved it got killed, some people who probably didn't got killed. Legion justice isn't in the habit of discriminating.