I think you're right - Locke saw Ben kill Keamy and presumably told Sun about it (not necessarily to get her to come back to the island, maybe more to get it off his conscience).

OK, let me rephrase that. I think that Locke told her so that she would look for Ben and then Ben could get the chance to get her back to the island by proving that Jin was still alive.

finally! i have been confused about why sun wanted to kill ben and haven't been able to figure it out. now it makes sense. what I'm wondering is if sun needs to go back to the island, what'll happen to her daughter? they only have a certain amount of time to go and her kid's back in Korea, will she just have to leave without her kid??

I doubt she'll leave without Ji Yeon. In fact, I don't think they'll return to the island until season 6 so she'll have plenty time to get her kid.

But why the 70 hours time limit then?

Maybe, they will spread it out. Like 24 is a day? I don't know, that's kind of crazy.

what i dont understand is y jack all of a sudden turned around and started working with ben. i have a question...
did Locke die after he turned the wheel to get off the island or what and who is that guys sun that was helping Locke get to the wheel.
is his sun Jack??????

Jack worked with Ben because his life is crap and he thinks that if he works with Ben and goes back to the island he will get his life together. Locke did not die after turning the wheel. He talked to the Oceanic 6 first. Yes, Jack is his son.