Jesus said this to challenge them. He wanted them to acknowledge that it is lawful to heal people on the Sabbath. (See: [[rc://en/ta/man/jit/figs-rquestion]])
It may be helpful to repeat "is it lawful," as that is the question Jesus is asking again in another way. Alternate translation: "is it lawful to save a life or to kill" (See: [[rc://en/ta/man/jit/figs-ellipsis]])