"You're asking me to be rational", "Women and children too", "Im sorry"

There is an excerpt from the Return of the Jedi novelization that state that the reason Vader stopped Luke from striking down Sidious is because Luke hadn't fully consumed the Dark Side. The Death Star is attacking the rebel alliance under Sidious' command and Luke's attempt to strike down Sidious was the only way to defend the alliance but Vader wanted Luke to be in touch with his hate and anger before he can do the job so he stepped in and clashed sabers with Luke.