Mo. Rev. Stat. § 273.030: Dogs may be killed, when.

If any person shall discover any dog or dogs in the act of killing, wounding or chasing sheep in any portion of this state, or shall discover any dog or dogs under such circumstances as to satisfactorily show that such dog or dogs has or have been recently engaged in killing or chasing sheep or other domestic animal or animals, such person is authorized to immediately pursue and kill such dog or dogs; provided, however, that such dog or dogs shall not be killed in any enclosure belonging to or being in lawful possession of the owner of such dog or dogs.