What fallacy is this

"Murder is good because it's sometimes justified"           

First off murder is to unlawfully kill, so can't be justified, but I think they meant kill

There is a difference between what is legal and what is justified. So it is possible they really did mean "murder". The statement is a basic non sequitur because it does not follow that murder is (overall / in general) "good" because of the isolated cases where it might be morally justified. To make this a reasonable claim, we would need to say:

"Murder is sometimes good because it's sometimes justified." 

Then, of course, the debate can be focused on if murder is ever morally justified or not.

“Murder” can be an ambiguous term. Sometimes people mean “killing that violates the law” but other times mean “killing that is morally unjustified”. It can even be used to draw distinction between an accidental killing and an intentional (murderous) killing.

Unfortunately we need to know more about the claim being made in order to ascertain if it’s fallacious. If the definition of murder being used by your interlocutor was “unjustified killing”, it would be known as a violation of the law of non-contradiction to state that “murder, AKA unjustified killing, is sometimes justified”, so the conclusion that it is ‘good’ would not be able to be reached.

However, if they meant something like: “Murder, AKA intentional killing, is sometimes morally justified. Morally justified things are always good to perform. Therefore, murder is sometimes good to perform” then there would be no fallacy committed (and dare I say your interlocutor would have a valid and sound argument!)

The implied argument would seem to be something like:

P1: Murder is the unlawful killing of a person

P2: Sometimes killing another person is justified 

C: Therefore, murder is good.

I see 3 issues:

problem 1 : P1 isn’t completely accurate because it’s incomplete.  There are other forms of “unlawful killing” besides murder so what applies to “unlawful killing” doesn’t necessarily apply to murder. 

problem 2 : jumping from “unlawful killing” to “some situations of killing”.  We have a non-sequitur because of undefined terms that can be given different meanings by different people.

problem 3 : another non-sequitur – jumping from something being “justified” to it being necessarily  “good”.

I’m not clear about where the argument is intended to go, unless the point is something like: 

As bad as killing is in general, there are some situations in which it can be justified, necessary or condoned.

if so, it’s less an argument and more a statement of opinion.

cherry picking