John Doom said:

I disagree: Vader killed the Emperor and his sacrifice actually made him turn BACK from the darkside :D

If the Jedi use the Force for defence, why couldn't Luke defend himself? He might have turned to the darkside only had he acted in anger (as the Emperor said).

Well he almost fell to the dark side when he fought Vader. At least by doing what he did he actually saved the galaxy, as he triggered Anakin's return. If he did what you suggest he would either die a completely pointless death or worse, become Emperor's tool.