I don't think Snape would ever kill her. As evil as he was made out to look, deep down he was a good person.
Now, I do believe that McGonagall could have killed him. More out of rage and hatred. I don't think she knew that Dumbledore had planned for snape to kill him, so not knowning this could have gave her enough courage/strength/rage to kill him. Especially since she was extremely fond, in a motherly way, towards Harry. Knowning that Snape was 'in on trying to get Harry' would have helped with getting enough rage to kill him. She wouldn't have killed him because 'she could' or because she was evil, more she would kill him because she knew he woul;d have caused more harm and pain to others
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