First off, whether or not this man "deserves to die" is irrelevant. You don't have the right to make that judgment and neither do I (that is, if you give any respect to what the Constitution says about fair trials and the justice system). And more importantly, it has nothing to do with the 2nd amendment (it actually has to do with the 6th Amendment, which guarantees a right to a fair and speedy trial). Even if we agreed that you had an unlimited 2nd amendment right to own any weapon ever made for any reason, that still wouldn't give you the right to kill someone in cold blood, because that violates the right of the accused to a fair trial. So if that's the reason you want a gun, so you can kill accused criminals in violation of their 6th amendment rights (as well as their right to life), then I hope you're ready to abandon your 2nd amendment constitutional right as well.
There is a man in Boston that went to jail a couple years ago. Apparently, this man had quite a criminal rep. Then one day, he decided, with a friend, that it would be a good idea to kill a boy. So they found one, and suffocated him with a gas-soaked rag. Then they stripped the body and had sex with it. Then they stuffed the body in a plastic box, filled the box with concrete, and chucked the box in the Charles River. This man doesn't deserve to die?
Second, I have a counterexample: one time, there was this woman who bought a bunch of guns. One of them was called the Bushmaster rifle. She bought them legally, and did not have a mental health record or history of violent crime, so she had no problem buying the guns. Her son Adam Lanza stole some of the guns, shot her to death with them, then shot a bunch of kids at a school nearby.
If guns were illegal, that mother would not have purchased those guns and Adam wouldn't have been able to use them to kill those kids. And you say gun control is bad?
Please note, I'm not actually using the above as a genuine argument for gun control. I'm just demonstrating how stupid and asinine it is to have this discussion in terms of anecdotal evidence. Gun advocates dig up some anecdotes of situations where guns supposedly could've saved someone's life, and that's supposed to override the evidence that gun control does drive down crime significantly when it's enacted properly, and that states and countries with "right-to-carry" laws don't actually correlate with decreases in gun violence:
Research, wherever it is done, indicates that higher rates of gun availability are consistent with higher rates of gun fatality, both accidental and homicidal. And so any real discussion of "the gun problem" in America must necessarily consider these statistics. Anecdotes may sound cute and they may be short and sweet and easily C+Pable and recitable on internet forums, but they don't take the place of actual facts and statistics.
The initial model specification, when extended to new data, does not show evidence that passage of right-to-carry laws reduces crime. The estimated effects are highly sensitive to seemingly minor changes in the model specification and control variables. No link between right-to-carry laws and changes in crime is apparent in the raw data, even in the initial sample; it is only once numerous covariates are included that the negative results in the early data emerge. While the trend models show a reduction in the crime growth rate following the adoption of right-to-carry laws, these trend reductions occur long after law adoption, casting serious doubt on the proposition that the trend models estimated in the literature reflect effects of the law change. Finally, some of the point estimates are imprecise. Thus, the committee concludes that with the current evidence it is not possible to determine that there is a causal link between the passage of right-to-carry laws and crime rates.
---National Research Council, 2004
For example, my grandpa was a cop for years; he has several anecdotes about drivers who got into horrible accidents, but survived solely because they didn't put their seat belts on. These would largely be considered freak accidents, fairly uncommon, but using your logic, I could easily cite them as evidence that wearing a seatbelt is actually deadly, and you should never wear one.
If by "the liberals" you mean "The Constitution," then I agree. The Constitution says that you have a right to a fair and speedy trial; the courts judge the degree of punishment for a crime, and we are legally and constitutionally bound to accept that judgment. If you violate that judgment, you are acting as a vigilante and you are violating that criminal's 6th Amendment right to a fair trial.
No, say the liberals. Rather, he is instead serving a prison sentence, and trying to legally change his name to some Wiccan babbling. And that's okay, say the courts.
EDIT: Also, you'd be in violation of Article III, Section 2 of the Constitution, which determines that "The judicial power shall extend to all cases, in law and equity, arising under this Constitution, the laws of the United States, and treaties made, or which shall be made, under their authority...The trial of all crimes, except impeachment, shall be by jury..."
I find it interesting that so many gun advocates also express such disdain for the court system; those types who would murder someone because of personal reasons are exactly the kind of people who should be deemed mentally unfit to own a gun, and yet they are the ones primarily lobbying for an individual gun right. How is it that the second amendment is non-negotiable and absolute, but the sixth amendment is negotiable if you don't like the guy? Which is it? Are you for constitutional absolutes, or do you think the Constitution gives people rights they shouldn't have and should thus be ignored? It wouldn't hurt to have some consistency here.
But it's not okay. If people had the right to shoot killers and criminals, this pathetic joke of a man would be dead. And this is bad?