No question about it - if the murderer is put to death, he/she won't be able to do it again. That is a fact. Stated that way, it has nothing to do with revenge, and everything to do with prevention.
If the issue could be thought through, one point at a time, it might make more sense. However, you immediately encounter the "Yeah, but I'm against the death penalty because..." opinions that prevent the discussion from going any further.
Might be interesting to see a 'scorecard' on this...at least it would show all the points for and against, and the reasons for each one. The point you're raising on prevention of future murders is a valid one, in my opinion.