Times of Strict Judgment

This seems to be the answer to the question on Rashi. When HASHEM was taking retribution on the firstborns of Mitzrayim, justice was being served, and so there was a global shift in the middah in operation. Din went into effect. As such, it was a very dangerous time. Now man – any man – would be judged with the system of din, and very few individuals would be able to pass as innocent. Therefore, the Klal Yisroel were warned, “Do not go out from your home.”

The destructive angel was given permission to act in a manner different than under normal circumstances. A person who might be innocent under the normal mercy system would now be found guilty and might warrant death. Because of that, the Jews were advised to avoid the situation.