(Cont.) Now, Mary is an ex-assassin so she should have been used to that kind of pressure, but given the fact that she hasn’t been one in a while, it’s possible she got nervous and made the wrong decision of shooting Sherlock.
This is quite possible! However, even if it’s true, it changes nothing.
Mary’s decision was between the following:
1: Don’t shoot Sherlock, and let John find out that she has lied to the point of not even being the same person he thought he married, and in addition that she is or was a professional assassin.
2: Shoot Sherlock (whether fatally or not), and continue letting John believe that she is someone completely different and, incidentally, allow him to continue to exist in ignorance of his potential danger due to being associated with her.
The only moral decision she could have made (which is a kind of funny thing to say about a character on a show where ALL the leads have now committed execution-style shootings) was to not shoot Sherlock and to thus allow her secret to come out. However, while the WAY that she shot Sherlock could arguably have been influenced by the pressure she was under, she makes clear that in no case would she have made a choice that let her secret come to light. She reiterated this to Sherlock in the empty house, and expressed not only complete lack of regret but also total and calculated willingness to do it again.