Why is the attempted murder of Alonso and Gonzalo not only evil but pointless?

Antonio and Sebastian are the only two who haven't fallen asleep. They decide that if they take this time and murder the sleeping Alonzo and Gonzalo; Sebastian, the king's brother will be heir. This plan is pointless because Alonso's son is onl believed to be lost in the storm, and also because Ariel will never allow it to happen.