Griefing is not tolerated on the server.
If the land was claimed you can provide coords of where these villagers where killed below and they can be restored and the player who caused the damage will be banned.
Anyone is able to kill mobs anywhere so this will be fixed but the damage done in your claim is your responability as you should be careful who you add.
If the area was unclaimed, staff cannot help you, however I would reccommend you claim the land by making a party /p create name and then claiming by /p claim.
Best of luck