Or HaChaim on Exodus 5:4/אור החיים על שמות ה׳:ד׳

1 ויאמר..למה משה ואהרון תפריעו את העם, He said: "Why Moses and Aaron to you deter the people, etc.?" After Pharaoh had first faced both the elders and Moses and Aaron and they had told him about the demands of the Hebrew G'd, Pharaoh now turned to Moses and Aaron telling them that he had no complaint against the elders who were obviously motivated by their fear of pestilence. His complaint was directed against Moses and Aaron on two counts. 1) Seeing that Moses and Aaron had brought a message from their G'd, it appeared that the decision was up to him. They had therefore not been entitled to threaten the people with punishment by their G'd for something that was not up to them but up to him! 2) If Moses and Aaron had told the people that they could simply leave Egypt without being granted permission, why had they come to ask his permission at all? Perhaps this is what our sages had in mind in our Midrash when they said: "why does the Torah write אתם and why the word למה? This may mean that Pharaoh asked why they came at all, and why they told the people in the name of G'd that they could leave. Pharaoh then turned towards the Israelites and told them: לכו לסבלותיכם, "go and do your work!"