Did I think Lucas regrets the way Palpatine died?

Not at all. Why would he? It was the natural end of the story at the time. As Darth Caliban said above, killing the Emperor any differently wouldn’t prevent producers and writers bringing him back years later by cloning or other explanations, simply because they hadn’t planned out later films following on from the story and needed a “big baddie”.

I wouldn’t be surprised if Lucas thought it was a mistake to bring the Emperor back in TROS, and in the way they did it?