You could possibly do it, but I think it would require a few prerequisites:
-A personal, emotive reason why they would want to kill of a whole people. As other posters have pointed out, enslavement en masse is far more rational. But people can be irrational.
-The person with a grudge must have unquestioned authority within the state, or there will be a lot of opposition to wholesale slaughter. After all, enslavement is the more economical action.
But against all non-citizens? No. Wouldn't happen.
EDIT: Wikipedia has this to say
"The Isaurians were brought partially under control (76–75 BC) by the Romans. During the war of the Cilician and other pirates against Rome, the Isaurians took so active a part that the proconsul P. Servilius deemed it necessary to follow them into their rugged strongholds, and compel the whole people to submission, an exploit for which he received the title of Isauricus (75 BC)."
I guess instead of compelling the whole people to submission, Servilius might convince people that the only way to ensure there isn't any Isaurian resistance is to ensure there are no Isaurians.