[Region Warrior]

Asked my son what he thought the other night.
You guys are correct.

He said; "did you not watch Return Of The Jedi?" with a

Yoda said he would turn if the didn't complete his training before saving(or not saving) his friends.

While fighting he was losing(or had) to Vader until he gave up Lea's identity. That's what pushed Luke towards the dark side(anger and fear to save her and friends from Emperor and Vader).

Then he said mentioned some lines from the emperor;

Your hate has made you powerful.
Strike me down with all your hatred, and your path to the dark side will be complete.
He tried but Vader blocked his strike.

After going gonzo and defeating Vader, looking at his hand, had actually realized how powerful he had became. Not that he felt bad for Vadoer.
When Luke refused to join the Emperor he was hiding the fact he had turned.
He realized Luke was now powerful enough to destroy him given the chance.
That's why he was killing him.

Makes perfect sense...