Answer

First, even if people commit an act that carries the death penalty (such as male homosexual intercourse), that requires evidence (two witnesses) and a decision by a duly authorized religious court. A private individual cannot take the law into his own hands. Second, actions of this kind, whose purpose is social influence, have to be judged by their results. It is clear that such an act would have only bad consequences, including with respect to the matter itself (preventing the transgression), and therefore there is no justification whatsoever for doing it.
In many cases, the act is done in the heat of the moment and not in order to achieve the hoped-for benefit. When it is done rashly, and when it is clear that it will not achieve its goal, these are indications that the act was not done with deliberation. Murder that is not committed out of deliberation is never a good deed. As I understand it, even the rule that “zealots may strike him” was not said in a way that would permit acting thoughtlessly (Phinehas asked Moses before deciding to do the act).
In short, it is a bad act from every possible perspective. Could it possibly be that anyone who, in our opinion, acts against God’s will—and even if he really is acting against His will—may be murdered? Murder too is against God’s will.
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Questioner (another one):
Following up on this question,
You wrote: “Murder that is not committed out of deliberation is never a good deed.” In the post about the shooting soldier, you said that one should distinguish between the act in itself and the motives behind it. For example: if someone were to murder Hitler not out of deliberation, and even with bad intentions (say, for money), would that make the act of murder into a not-good act? (Likewise regarding Jael and Sisera.)
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The Rabbi:
Absolutely. It would be a bad act with good results. A moral act is always one done with deliberation and not as an instinct. Instinct is an animal reaction. See the fourth notebook, part three.
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Questioner:
In the comments on the post about the shooting soldier, it seemed that you said otherwise. Here is the quote:
 
“This is similar to a court sentencing a person to death, and the executioner kills him because he hates him. Is the executioner’s act invalid? In my opinion, no. His motives may be invalid, but not his act. This is a flaw in personality, but not a transgression in the act. A parable for the matter is what some later authorities wrote (the Netziv and Rabbi Kook), that a transgression for the sake of Heaven is permitted only where the transgressor’s intention is for a commandment. I disagree with them absolutely. The question is whether the act is justified, not what his intention is. If the act is justified under the circumstances, then even if he did it for another reason, the act is correct and permitted and desirable to do.”
“The soldier is judged for his act and not for his intentions. Only the reverse is true: if he were to do a criminal act with non-criminal intentions, then he would bear no criminal guilt. But there is no room to accuse someone of wrongdoing for a correct act done with criminal intent.”
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The Rabbi:
When the executioner kills the person because he hates him, he is doing a correct act with authorization, but alongside that he has hatred in his heart. Here we are talking about an act that, even if it is correct, was done without authorization and not out of a considered decision.
And even if this act is positive (assuming for the sake of discussion that it really is positive—I don’t think so), it was done with criminal intent, because killing without an informed decision is murder—exactly the opposite of a negative act done without criminal intent.
In the discussion about the soldier, the assumption is that he decided to kill the terrorist deliberately and did not do so in the heat of the moment. Still, on the legal level he did it without authority, and there is no legal defense for him, but such an act is, in my view, definitely moral.