In fact, it's the user of the term "serial killer" that is manipulative and dishonest. Per Wikipedia: "A serial killer is a person who murders three or more people, usually in service of abnormal psychological gratification, with the murders taking place over more than a month and including a significant break (a "cooling off period") between them." Amy didn't have a cooling off period, and she did not kill for psychological gratification. She killed for survival.
If anything, Amy is a spree killer (per Wikipedia; "in which murders are committed in two or more locations, in a short time"). But because the term "spree killer" doesn't inspire the same knee-jerk horror as "serial killer", fans who believe Amy Pond should have been killed usually avoid the more accurate term. In this case, while projecting their own dishonest manipulative tactics onto the show writers.
I've seen at least two fans argue Amy should be killed by pointing out how real-world justice would have handled her kills (discounting the huge controversy over the death penalty). Whether as a defense or an accusation, I don’t think I have *ever* seen *anyone* bring up real-world justice with any other character on SPN, despite the multitude of murders that characters on the show, including Our Heroes, commit. What is up with that?
And the reality is, Amy's situation is a very fantastic situation that it is difficult to apply real-world justice to.
Because Amy Pond does not end lives for the thrill of it. She has a biological need to kill. And more to the point, for the killings she committed in The Girl Next Door, Jacob has a biological need to kill.
Within the context of Sam and Dean’s other choices, I find Dean’s decision to kill Amy unambiguously appalling. I've outlined the reasons for this many times.
But even taking it as a moral dilemma disconnected from Sam and Dean's actions. It’s important to remember what it means for Amy to kill a human being. I’ve heard many people argue that Amy had to die because Jacob could get sick again, and then Amy would kill again. Okay…but if Amy’s dead, Jacob could still get sick. If Amy is a Bad Person who kills because she is Bad, and Jacob is an unformed and innocent child with the potential to choose Good (although not innocent enough to be spared witnessing his mother’s death and a death threat from the guy who killed her, apparently…), then I guess killing Amy sorta makes sense.
But something much of fandom and Dean himself (what with the aforementioned death threat and everything) fails to realize is that it’s not a matter of Jacob choosing not to be a killer. It’s a matter of Jacob killing a member of the species responsible for killing his mother, or dying. Probably a slow death. Possibly painful. So unless you got REALLY good reason to think that Jacob will choose to die rather than kill human beings like the one who killed his mother in front of him and threatened his life...
And killing Amy just makes it more likely that Jacob will be in the position, of choosing to kill or die.
If you kill Amy, you have to kill Jacob.
Unless you have some way of guaranteeing that Jacob will choose fatal illness and starvation over killing…if you want to make sure no people die, Jacob must die along with his mother.
I can’t picture Jacob nobly starving to death rather than feeding. Making a choice to die of starvation is also very physically difficult. Ordinary people have done horrifying things to keep from starving to death. It is nothing like killing for psychological reasons.
Being biologically dependent on eating your prey is simply not the same as being a thrill killer. Regardless of whether you think it’s more or less morally right to kill for biological survival, the two types of killing have to be dealt with differently because the causes and motivations are different. And if you treat a prey killing like a thrill killing, you will probably end up getting more people killed. As Dean probably did.
Because clearly, threatening to kill someone if they kill when the most likely reason they will kill will be because they are literally *dying*…I mean, what?