Originally posted by Ushgarak
Now, that is a debatable issue. There are three answers really, both related to recent events:1. Lack of anger. Vader isn't THAT pissed because he feels he just did rather well- he won the confrontation with his son, made the revelation he wanted to make and feels he may yet destroy the Emperor with Luke's help.
2. Whim. Vader feels what has happened is just the way it went, and this time doesn't feel that vindictive. Bad guys are prone to whims
3. Emotion- Vader is so uset at Luke gewtting away that he doesn't even feel like killing.
The most superficially attractive idea is that Vader is feeling pangs of remorse due to contact with Luke. I would avoid that idea though- it ascribes motives to Anakin which do not actually appear until the final confrontation.
Best to ascribe it to a combination of the above; one way or another it made for a very great dramatic moment.
I'd have to say this best nails the point. Evil badguys kill others because of all sorts of reasons, both legit and otherwise.