Against my people, i.e. against me acting for my people. The gracious God takes what is done to or against his people as done to or against himself. See Malachi 2:8 Matthew 25:40,45 Ac 9:4,5.
Related Commentaries of Exodus 9:17
Exodus 9:17
17 As yet exaltest thou thyself against my people, that thou wilt not let them go?