But Luke wasn’t planning to appear before the Emperor - Vader took Luke to Palpatine when he initially refused to turn back to the light side as Luke asked him to. Luke wanted to surrender to the Imperials to avoid having Vader sense what the Rebels were doing through him, and to have a face-to-face talk with his father. If he failed at turning his father, he might be killed, but his friends would no longer be in danger from his Force link to Vader.

And besides, why would Luke think it was his job to kill the Emperor anyway? The Rebel fleet destroying the Death Star was supposed to be responsible for that.