While the murders of twelve people at Charlie Hebdo were morally unjustifiable, one of the killers did attempt, in advance, to justify the attack. The justification offered was that the attack was in accord with Islamic law. Since I am not a scholar of Islam, I will not address the issue of whether this is true or not. As an ethicist, I will address the matter of moral justification for the killings.
From the standpoint of the killers, the attack on Charlie Hebdo was presumably punishment. In general, punishment is aimed at retaliation for wrongs done, to redeem the wrongdoer or for deterrence (this is the RRD model of punishment). Presumably the killers were aiming at both retaliation and deterrence and not redemption. From a moral standpoint, both retaliation and deterrence are supposed to be limited by a principle of proportionality.
In the case of retaliation, the punishment should correspond to the alleged crime. The reason for this is that disproportionate retaliation would not “balance the books” but instead create another wrong that would justify retaliation in response. This, of course, assumes that retaliation is justifiable in general, which can be questioned.
In the case of deterrence, there is also a presumption in favor of proportionality. The main reason is the same as for retaliation: excessive punishment would, by definition, create another wrong. A standard counter to this is to argue that excessive punishment is acceptable on the grounds of its deterrence value on the view that the greater the punishment, the greater the deterrence.
While this does have a certain appeal, it also runs counter to common moral intuitions. For example, blowing up a student’s car for parking in a faculty parking space at university would deter students, but would be excessive. As another example, having the police execute people for speeding would tend to deter speeding, but this certainly seems unacceptable.
There is also the standard utilitarian argument that excessive punishment used for deterrence would create more harm than good. For example, allowing police to summarily execute anyone who resisted arrest would deter resistance, but the harm to citizens and society would outweigh the benefits. As such, it seems reasonable to accept that punishment for the purpose of deterrence should be proportional to the offense. There is, of course, still concern about the deterrence factor. A good guiding principle is that the punishment that aims at deterrence should be sufficient to deter, yet proportional to the offense. Deterring the misdeed should not be worse than the misdeed.
In the case of the people at Charlie Hebdo, their alleged offense was their satire of Mohammad and Islam via cartoons. On the face of it, death is a disproportionate punishment. After all, killing someone is vastly more harmful than insulting or offending someone.
A proportional response would have been something along the lines of creating a satirical cartoon of the staff, publishing an article critical of their cartoons or protesting these cartoons. That is, a proportional response to the non-violent expression of a view would be the non-violent expression of an opposing view. Murder is obviously a vastly disproportionate response.
It could be replied that the punishment was proportional because of the severity of the offense. The challenge is, obviously enough, arguing that the offense was severe enough to warrant death. On the face of it, no cartoon would seem to merit death. After all, no matter how bad a cartoon might be, the worst it can do is offend a person and this would not warrant death. However, it could be argued that the offense is not against just anyone but against God. That is, the crime is blasphemy or something similar. This would provide a potential avenue for justifying a penalty of death. It is to this that I now turn.
Religious thinkers who believe in Hell face the challenge of justifying eternal damnation. As David Hume noted, an infinite punishment for what must be finite offenses is contrary to our principles of justice. That is, even if a person sinned for every second of their life, they could not do enough evil to warrant an infinitely bad, infinitely long punishment. However, there is a clever reply to this claim.
In his classic sermon “Sinners in the Hands of an Angry God”, Jonathan Edwards says of sinners that “justice calls aloud for an infinite punishment of their sins.” He justifies the infinite punishment of sin on the grounds that since God is infinitely good, any sin against God is infinitely bad. As such, the punishment is proportional to the offense: infinite punishment for an infinitely bad crime.
It could be contended that creating cartoons mocking Mohammed and Islam are sins against an infinitely good God, thus warranting an infinite punishment and presumably justifying killing (which is much less than infinite punishment). Interestingly, the infinite punishment for sins would render punishing of sinners here on earth pointless for two reasons. First, if the sinner will be punished infinitely, then punishing him here would not increase his punishment. So, there is no point to it. Second, if the sinner is going to be punished divinely, then punishment here would also be pointless. To use an analogy, imagine if someone proposed having a pre-legal system in which alleged criminals would be tried and, if found guilty, be given pointless sentences (such as being mildly scolded). The alleged criminals would then go on to the real legal system for their real punishment. This pre-legal system would be a pointless waste of time and resources. Likewise, if there is divine justice for sins, then punishing them here would be a pointless waste of time.
This, obviously enough, assumes that God is real, that He punishes and that He would punish people for something as minor as a cartoon. Attributing this to God would present him as a petty and insecure God who is overly concerned about snarky cartoons. People are most likely to react violently to mere mockery when they are strong enough to punish, but weak enough to be insecure. God should not be enraged by cartoons. But I could be wrong. If am, God will take care of matters in the afterlife and there is thus no reason to kill cartoonists.
If God does not exist, then the cartoons obviously cannot have offended God. In this case, the offense would be against people who believe in fiction. While they might be angry at being mocked, killing the cartoonists would be like enraged Harry Potter fans killing a cartoonist for mocking Daniel Radcliffe with a snarky cartoon. While they might be devoted to the world of Harry Potter and be very protective of Daniel Radcliffe, offensive cartoons mocking a real person and a fictional world would not warrant killing a cartoonist.
As such, if God is real, then He will deal with any offense against Him. As such, there is no justification for people seeking revenge in His name. If He is not real, then the offense is against the make-believe, and this does not warrant killing. Either way, such killings would be completely unjustified.