“We all were sea-swallow’d, though some cast again,
And by that destiny to perform an act
Whereof what’s past is prologue, what to come
In yours and my discharge.” (Act II, Scene 1, Line 239-242)
This quote is said by Antonio in Act II, Scene 1, when everyone from the shipwreck has fallen asleep on the island and Antonio and Sebastian are the only ones who are awake. Throughout their conversation, Antonio is trying to convince Sebastian to kill his own brother, Alonso, the King of Naples, in order for him to inherit the power. This plan is able to work is because with Alonso out of the picture, his daughter residing too far away to obtain the rule of Naples, and his son’s all but certain death, Sebastian is guaranteed to succeed the crown and take over his brother’s kingdom. Taking advantage of the circumstance surrounding Alonso’s daughter Claribel, Antonio was able to persuade Sebastian, saying that she was the cause of the shipwreck, and that her fate was destined to assist them in seizing control. In relevance to the play, the quote serves as a crucial juncture in shaping the plot of the story and the events to come, and in this foreboding establishes their character and the tone for the rest of the story. In doing so, the story gives credence to the repercussions of greed and unfounded ambition.
I agree with you and understand why you think Antonio is trying to convince Sebastian to kill his brother so he can inherit the crown. This makes sense because as you mentioned, if Alonso is gone there would be nobody else to take his place other than Sebastian. This would end up giving Sebastian the easiest way to power. Unfortunately, given Claribel’s position it was quite easy for Antonio to convince Sebastian to kill Alonso. This absolutely does show the potential issues one can encounter when too ambitious/greedy as you mentioned.