Couldn't he just kill them?

If Prospero has all of Ariel's powers at his disposal, why are the tasks he asks Ariel to carry out for him so small? Why put people to sleep to eavesdrop on your enemies' conversation, play mysterious music here and there, mimic voices to cause drama in a group, and threaten enemies in the form of a harpy? Why is that the lead-up to the perfect revenge plan? Wouldn't it be easier for Prospero to get his vengeance simply by killing his brother? Or maybe just stealing the boat and stranding him on the island? But, no. That would make too much sense.