Count Dooku had his head cut off by Anakin, I guess that means he's still alive somewhere.
The rule is if the sith character was visibly executed in a way that guarantees death, they're dead. Darth Maul had his legs cut off, that doesn't include any important organs. Disney is just being stupid bringing back Palpatine, he was supposed to be dead on the Death Star.
If the character is a Jedi, they die very easily as evidenced by Qui-Gon Jinn, all the Jedi killed by order 66, Obi-Wan, Yoda and Luke.