Look at Act 2, Scene 1, Lines 269-297 of The Tempest. Consider how Shakespeare portrays Sebastian.
In this section of the scene Antonio convinces Sebastian that he should kill his brother and become King. Sebastian, to his credit, initially is rather dismissive of the idea, referring to the conscience of Antonio as a reason for not killing his brother and gaining the crown. However, at the same time, Antonio manages to persuade him very easily that he should ignore such moral scruples and that Sebastian has far more to gain than he could ever lose through his actions. Note how Sebastian eagerly agrees at the end of this section:
Thy case, dear friend,
Shall be my precedent. As thou got'st Milan,
I'll come by Naples. Draw thy sword. One stroke
Shall free thee from the tribute which thou payest,
And I the King shall love thee.
Sebastian is therefore portrayed in these lines as a treacherous, evil and unscrupulous individual. He is easily tempted to commit an act that is morally wrong, but is only willing to try and kill his brother when he is asleep and unable to defend himself. This speaks of a moral cowardice that is reprehensible.
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