I'm going to say this might be what in Syllogistic logic would be called a very subtle fallacy of necessity, and if so, it's actually a very neat example!
Let's try to phrase the argument formally like this:
- All murders are crimes.
- No Crimes occur when they are legalized.
- Therefore, no murders will occur when they are legalized.
The problem with this is that the second premise seems tautological - it's just a simple matter of semantic fact that anything that's legal isn't a crime. But this doesn't extend to murders being crimes. It's not simply semantically tautological that a murder is a criminal act - it has to be a matter of legal fact that says that it is indeed illegal to commit murder. But the conclusion doesn't take this specifically contextual aspect of the first premise into account. What will happen when murder is legalized is that it will cease to be a crime, and that the second premise, while still true, will cease to be contextually relevant.
As discussed in the Wikipedia article, what seems to be going on is that there is an equivocation between the De Dicto necessity of premise 2 (something that is simply logically necessary) and the De Re necessity attributed in the conclusion 3 (something that is a proposed necessary property of acts of murder). If we were to interpret the necessity of premise 2 in a De Re manner (asking about necessary properties of acts that are criminal), we would simply say it was false - it's just not necessarily true of anything that in fact is a crime that it wouldn't happen any more if we legalized it.