I find this motive somewhat exasperating. It trades so often on manipulating audience sympathy rather than on reality or rationality.
I recently had the unpleasant experience of being reminded how much like high school online communities can be. I commented on Columbophile about the episode "Try and Catch Me" in which Ruth Gordon plays the murderess. She kills the apparent murderer of her niece, the widowed husband.
Columbophile makes the strong argument that the original crime is somewhat ambiguous. The nephew-in-law may have murdered the niece; he may not have. I complimented Columbophile for the insight. I always felt the same when watching the episode, and I thought it was an interesting twist. The aunt believes her nephew-in-law is a murderer and acts accordingly, but the ambiguity raises the problem of how far anyone should really go to save even family--that is, past the point of definite proof.
Like many online communities, Columbophile's community is apparently comprised of people who don't allow for interpretations other than the one they have all agreed on! It's okay for the blog master--the blog master exists so they can disagree or agree with someone else's labor in order to formulate how the group is supposed to think. I was consequently informed in quick succession by several commenters that the episode is NOT ambiguous AT ALL. Since I've seen the same thing happen on Althouse, I reminded myself that most online communities revert to high school "pack" behavior and returned to my non-subscription observation status.
Since I think murder is wrong even for obvious motives, I never felt the need to justify the aunt. Hence, I was able to enjoy Ruth Gordon's masterly performance without giving up the ambiguity. (Ambiguity is not the kind of thing that one simply decides doesn't exist--this is one place where the deconstructionists are right: all language is inherently ambiguous. But then many people appear incapable of handling contradictory material within a single text or work.)
It also illustrates the emotional power that the justification I'm revenging/protecting family can have--explaining why "kissing babies" is such a common behavior by institutions. Of course, there is a gap between kissing babies and actually helping babies, so even my church--which defends motherhood and children, etc. etc.--makes all the right noises about babies but spends more time worried about teens (don't all Americans?).
One of my few favorite mysteries that uses this motive--I'll do anything for my family--is the 2-parter in Castle where Alexis is kidnapped. At the end of Part I, Castle confronts one of the kidnappers. He softly asks the kidnapper for information. When the man refuses, Castle closes his eyes, then opens them to reveal the ruthless side of Castle that audiences more often see in Mal and only very, very rarely with Castle. There's no justification provided for Castle's willingness to torture the man for information. It simply is a choice and--considering Castle and his daughter's exclusive relationship for much of their lives--makes sense. Of course, shows like Supernatural rather delightfully take the premise I will do anything for family to the most extreme degree: Would you? Would you really? Make a deal with Satan? Go to literal hell? Sacrifice yourself on an altar to demons? Ah, you think you would, but would you really?
Face it, folks, you aren't Dean and Sam!