If she shrieked for help there was very little chance of anyone hearing her, whereas there was probably quite a good chance of Mrs. Vandemeyer's shooting her.
Mrs. Vandemeyer might hit her, then shoot her. 
mnli hypothesis: Mrs. Vandemeyer might hit her, then shoot her.  premise: If she shrieked for help there was very little chance of anyone hearing her, whereas there was probably quite a good chance of Mrs. Vandemeyer's shooting her.