That whole "Emperor could have been redeemed" theory is even more hypothetical than my "Lives saved by Emperor's death" theory. But the chances of the Emperor destroying many lives are greater than the Emperor being redeemed, why would he build another Death Star.
You can apply your argument to Obi Wan. Why does he get to chill with Yoda afterlife? He killed Darth Maul with the advantage of his good looks and Scottish accent. He also b1tch slapped Annakin into becoming Darth Vader. He's was indirectly responsible for Alderaan being blown up.
Hell, does Luke get to go? He has quite an impressive kill count as well (chalk up one Death Star full of fathers and sons).
But then you'd prolly say "Well those guys were evil..." or "blah blah blah blah blah."
Seriously, this discussion can involve many many psycological and sociological factors. All I can say is that it is a movie created to entertain people, and I think it pretty much worked on normal people.