Sasuke stabbed Karin and then told Sakura to kill her because she was worthless to him.

If she went back to him in real life, she would be classified as an abused woman. Having her forgive him sends a bad message to kids everywhere. Judging by the way he has consistently sidelined female characters since the beginning of part 2, I know Kishi's probably not thinking of the girls reading this (I don't think the realize how large a part of his fanbase they are). However what kind of message does this send to the young boys? That if you are a cool guy its okay to betray girls who are completely devoted to you, so long as you apologize to them?
We can say things like "oh, Karin's a mess, she's always been a mess" but will the younger kids be thinking that? This is a manga that has always been for adolescent boys, no matter how much teens, adults and girls enjoy it. Its like Harry Potter. And therefore like JK Rowling Kishi has to be careful of the subliminal messages he's sending. The reason so many people are disappointed with him on this point is because it feels like he's treating a very delicate subject carelessly.
As far as Sasuke turning good goes, I'm dubious about that sticking or even being real at all. In fact, my first impression when I read it was that Kishi was trying to pull a fast one on us, but it was so out of left field no one was going to believe it.
Oh, how I was wrong.
Not only do plenty of people believe it, there are many people on the forums who
are glad this happened. They are calling it an "epic chapter" and "the best chapter." I can only hope these are the aforementioned twelve year old boys coming to this conclusion.
Logically, I doesn't make any sense. Forget all the hints about an epic Sasuke and Naruto confrontation, I'm a writer who has studied plot extensively and having Sasuke turn suddenly good wouldn't make any sense for the plot. Right now, we've got two definite bad guys: Obito and Madara. Meanwhile on the side of good we have
the entire shinobi world, including Naruto, who is supposed to be the greatest shinobi since the First Hokage. For some reason I can't fully comprehend, this is an even fight. (So much for the new generation surpassing the previous). But since Madara is the grand-daddy of all Uchiha and the greatest supervillain in Naruto history, and Tobi is his protege, I'll buy it sort of.
Now for a finale, Naruto needs an opponent who is as great as him, who no one else can fight, to fight on his own. So lets say he gets either Obito or Madara. That leaves the other one to face
the entire shinobi world, which is a battle they will hopefully lose (
if not, see this TvTropes page). Suspending my disbelief, let's just say this is a difficult battle, worthy of being the final fight of the series, but our heroes pull through. It is a decent ending, but not worthy of a story of this scale. There needs to be a moment, a crisis, where
all hope is lost and it genuinely looks like the hero will not pull through. The odds need to be turned against the hero, all hope needs to be lost and then the hero does something incredible and miraculously wins the fight. And honestly, this is a pattern that Kishimoto follows pretty consistently for Naruto's fights.
So let's say Sasuke comes along with Orochimaru and his merry band of Hokages.
And they join the entire shinobi world against Madara and Obito. The First is supposed to be as strong as Madara. So what, its Naruto, Sasuke, Orochimaru, three Hokages and
the entire shinobi world against Obito now??? At least when Kabuto was around there were zombies to keep
the entire shinobi world occupied.
Something's going to shift in the favor of evil and my bet is that Sasuke and/or Orochimaru are not suddenly all good and buddy-buddy with Konoha. The First is out of Orochimaru's control and could probably kill them all. So why not bring him to the battlefield where Madara can occupy his attention? I can see it in my head easily: "Oh look, Sasuke and Orochimaru are good and brought us zombie Hokages as presents how thoughtful - WHY IS THE FOURTH TRYING TO KILL KAKASHI???"
It makes sense. Evil gets Sasuke, Orochimaru, and the three zombie Hokages while Good gets the First Hokage. Its enough to tip the scales in the favor of Evil but with the addition First Hokage who can fight against the biggest threat (Madara), the odds aren't impossible.
Naruto is about underdogs. If Orochimaru and Sasuke joined the side of good, Madara and Obito would be extreme underdogs.
Even if they aren't tricking the First so they can get to the battlefield, something more is going on. Look beneath the underneath.

p.s. Why don't we have a Kakashi emoticon?