Several scenarios honestly, remember, Mystique is probably the worst person you could ever know.
Some reasons she would want to kill Null:
- For fun
- Because she is bored
- Because Rogue likes him
- Because Kurt likes him
- Because she thinks killing him will help her reconnect with her kids
- Because her girlfriend said at an undisclosed time in the future, Null may or may not be in the proximity of an event that may mildly inconvenience either herself, said girlfriend or both
- Because she might be batshit insane
Hence why I advocate to make her Willie E. Coyote, it would be hilarious.