For a chance to defeat the Emperor. And I don't think he pretended, he succumed to the dark side hoping he could eventually be redeemed like his father after defeating the Emperor. He used dark side powers of subversion and manipulation even on Liea. To use an analogy he became a traitor to the New Republic, in order to finally defeat the EMpire from the inside.
I think Darth Whiny would have been a good title
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get busy with it!