Barnes observes in commenting on Mark 3:4 -
It seems to have been a maxim with the Jews that not to do good when
we have an opportunity is to do evil; not to save life is to kill or
to be guilty of murder. If a man has an opportunity of saving a man's
life when he is in danger, and does not do it, he is evidently guilty
of his death.
Thus, when Jesus asked the question, to which the Jewish maxim was the obvious and unavoidable answer, "Which is lawful on the Sabbath: to do good or to do evil, to save life or to kill?" they remained silent; precisely because to answer would have betrayed their duplicity; the Jews were plotting to kill Jesus. Thus, to answer, "to kill life" would have made them guilty of disregarding the Torah; to answer "to save life" would make them guilty of the death of Jesus whom they wanted to kill for saving life.
By this ingenious question Jesus exposed what happens to people who, though punctilious about keeping the law actually end up breaking the law, in this case the Sabbath.
Jesus' question is a classic case of argument from the greater to the lesser. If the Jews were willing, on the sabbath, to rescue an animal or person from danger and death, surely the lesser act of healing a person would not be a crime on the Sabbath.
Secondly, while the Jews were willing to save an animal from danger and death on the Sabbath, they were quite willing to plan Jesus' death on the Sabbath!
Thus, they remained silent and Jesus was both angry and sad (Mark 3:5). Their silence was also significant. As Matthew Henry observes:
But stubborn infidels, when they can say nothing against the truth,
yet will not yield.
Thus, the Jews condemned themselves.