Actually it's one of the best parts of the movies ever. There's plenty of lead up - Yoda tells Luke not to underestimate the powers of the Emperor, Luke knows he has to confront Vader, the Emperor shows Luke that the rebels are losing and that he can save them by turning, he draws attention to Luke's weapon, both the Emperor and Vader taunt and persuade and attack him the whole time, and finally, Vader goes for where he is most vulnerable, with his care for his family. It's perfect as it is, don't cheapen it with Lucas-level obviousness.