In Macbeth, why does Hecate plan to kill Macbeth?

She does it because she is evil--she's the goddess of witchcraft, and in Shakespeare's day, witchcraft was firmly associated with devil-worship. Remember that Shakespeare wrote Macbeth after the death of Queen Elizabeth, in the reign of King James I, who (1) was Scottish and (2) was very concerned about witchcraft. James had also become Shakespeare's patron. So the play was pretty clearly written to please the King, both by showing him as a descendant of Banquo and by depicting the evils of witchcraft.

So, then, one could say that Hecate's objective is to win a soul for Satan. Once Macbeth has become so morally corrupt that even the witches themselves refer to him as "something wicked," all that remains is to kill him while he's in such a bad spiritual state.