In the fifth edition, Hurley gives the same example and apparently considers "all abortions are murders" to be a suppressed premiss of the argument. So, Hurley takes the complete argument to be:
Murder is morally wrong.
All abortions are murders. (Suppressed)
Therefore, abortion is morally wrong.
This is certainly a valid argument. Moreover, it doesn't appear to be circular, since the conclusion is not one of the premisses. Why, then, does it beg the question?
It begs the question because the word "murder" is not a morally-neutral word, such as "killing". All murders are killings, but not all killings are murders. A person who kills someone in self-defense, a soldier who kills in battle, or a policeman who kills in the line of duty, is not a murderer. So, the first, unsuppressed premiss is really unnecessary, as the argument is valid without it:
All abortions are murders.
Therefore, abortion is morally wrong
.
Or, to spell out what "murder" means:
All abortions are wrongful killings.
Therefore, abortion is morally wrong.
Which is clearly circular and, therefore, valid. In the original argument, the moral wrongness of abortion has been smuggled into the premiss via the morally loaded word "murder". This is how real-life questions are often begged, that is, by using loaded language to conceal the fact that an argument is circular.