Here's some options.
- The genie is a slave: it resents being used for its abilities by punny mortals, so it screws them over however it can.
- The genie is alien: it doesn't understand what humans actually mean, so it takes them literally when it interprets their wish.
- The genie is malicious: it wants to hurt you and probably escape its confinement, or sees you as a tool to do evil.
- The genie is a pedant: it deliberately only grants the letter of the wish, regardless of consequences, for whatever reason.
- The genie is a dick: he doesn't like you and enjoys goading, trolling, and generally fucking with you for its own amusement.
- The genie is just following the rules: whatever force traps the genie requires it to be like this. Maybe whatever trapped it decided, or the rules of god, its society, or its nature force its behavior.