Luke did have potential to be a good Dark Jedi. He obviously was jealous of Han's relationship with his sister and a hate-fuelled lightsabre attack on both of them would have cemented his place as one of best Dark Jedi ever. In fact, at the end of RotJ I still get a feeling he was a bit miffed about it all.
However, destroying Leia, Han and the Rebel Alliance would not have put rest to the Skywalker story and I see no way that the Father & Son alliance could ever be defeated and of course the prophecy would not be fulfilled. I'd stick with Luke being on the lightside.