The short answer is: Because he sacrificed himself out of love for his son, who had just spared his life when he was defeated and helpless.
For the full answer, we need to begin with a little background.
When <PERSON> is preparing to surrender to <PERSON>, he stops to tell <PERSON> the good news ("I'm your brother!") and the bad news ("<PERSON> is our dad!"):
<PERSON>: Then you know why I have to face him.
<PERSON>: No! <PERSON>, run away, far away! If he can feel your presence, then leave this place! I wish I could go with you.
<PERSON>: No, you don't. You've always been strong.
<PERSON>: But why must you confront him?
<PERSON>: Because there is good in him. I've felt it. He won't turn me over to the Emperor. I can save him. I can turn him back to the good side. I have to try. [kisses <PERSON> on the cheek, then leaves]
<PERSON> finds an Imperial soldier and surrenders; he is taken to <PERSON>, but the family reunion doesn't go quite the way <PERSON> had planned:
<PERSON>: The Emperor has been expecting you.
<PERSON>: I know, father.
<PERSON>: So, you have accepted the truth.
<PERSON>: I've accepted the truth that you were once <PERSON>, my father.
<PERSON>: [angry retort] That name no longer has any meaning for me.
<PERSON>: It is the name of your true self. You've only forgotten. I know there is good in you. The Emperor hasn't driven it from you fully.
[steps off to the side of the passageway, away from <PERSON>]
That was why you couldn't destroy me. That's why you won't bring me to your Emperor now.
<PERSON>: [inspects <PERSON>'s lightsaber] I see you have constructed a new lightsaber. Your skills are complete. Indeed you are powerful, as the Emperor has foreseen.
<PERSON>: Come with me.
<PERSON>: <PERSON> once thought as you do. You don't know the power of the dark side! I must obey my master.1
<PERSON>: Your thoughts betray you, Father. 605
I feel the good in you, the conflict.
<PERSON>: There is no conflict.
<PERSON>: You couldn't bring yourself to kill me before and I don't believe you'll destroy me now.
<PERSON>: You underestimate the power of the Dark Side. If you will not fight, then you will meet your destiny.
<PERSON>: I will not turn...and you'll be forced to kill me.
<PERSON>: If that is your destiny.
<PERSON>: Search your feelings, father. You can't do this. I feel the conflict within you. Let go of your hate!
<PERSON>: It is too late for me, son.2 The Emperor will show you the true nature of the Force. He is your master now.
<PERSON>: Then my father is truly dead.
There are two crucial lines here, but they are very easy to overlook:
1 <PERSON>'s choice of words here is important. He says "You don't know the power of the Dark Side. I must obey my master!" Why is this important? Because it means that <PERSON> doesn't obey <PERSON> because he likes him, or because he wants to do it. He obeys because the Dark Side is too powerful to resist. He is trapped.
2 Again, when <PERSON> says "Let go of your hate!", <PERSON> doesn't say "No way! Hate is the best thing ever, and I love it!" He says, rather mournfully, "It is too late for me, son. The Emperor will show you the true nature of the Force. He is your master now." This gives me the impression that:
* <PERSON> regrets becoming a Sith. He doesn't like being <PERSON>. He feels that his bad decisions in the past have doomed him to remain evil, and if he had the opportunity, he would go back and do things differently.
* If <PERSON> could leave the Emperor's service, he would do so, but he thinks he isn't strong enough to pull it off, or good enough to deserve a better life. He hates himself.
Moving on:
<PERSON> is brought before the Emperor, and <PERSON> provokes him into losing his temper and attacking <PERSON> in blind rage. 605