The St. Petersburg Mary was executed for more than just the murder. Peter th Great had been making an effort to reduce the level of licentiousness leading to unwed motherhood and to build up the population. Mary had repeatedly murdered her infants and, after she had been warned, did it again. Despite her involvement at court, Peter had no choice but to execute her to demonstrate that his laws applied to all, both high and low.

There was a book about Peter a couple of years ago which mentioned this. In an earlier thread on this topic, I gave the exact citation.