I think it depends on your intentions. I like to sword fight in lucids. They usually end with me "killing" a DC. But my intention is not to kill the DC. It's to experience the sword fight. If I told the decapitated DC to stop being a pussy and put his head back on he would. And we'd laugh about it afterwards. No harm done.

Now if you're goal is to simulate killing for the sake of killing I would consider that unethical. Summoning that annoying guy from school and slowly disemboweling him seems like the wrong thing to do.

But in the end you can no more kill a DC than you can kill a rock. If killing a DC makes you feel bad then don't do it. If you enjoy killing them just for the sake of killing then check yourself.