Rab Judah the son of R. Hiyya said: This teaches that Cain inflicted upon his brother many blows and wounds, because he knew not whence the soul departs, until he reached his neck.
We can see that Cain did not know how to kill before he killed Abel, though perhaps you could say that he only didn't know how to kill humans, although the Talmud says that he hit him a bunch of different places before reaching the neck. If he already knew how to slaughter animals, wouldn't he have started at the neck?
Rabbi Joseph Albo says in Sefer Ha'Ikarim That Cain thought that animals and humans were on the same level and consequently that animals were not to be killed. Abel had a similar thought process, with the exception that animals could be killed for the sake of Gd.
So it is evident that neither Cain nor Abel killed animals either up to this point, when they were told by Adam to bring sacrifices. If Abel had actually slaughtered his sacrifice, Cain would have at least had the idea to strike Abel in the neck and kill him, especially since he thought that humans and animals were equatable.
Based on this, I would argue that Abel did not kill his sacrifice, because no one knew how!
(I assume that Cain was present at Abel's sacrifice, to see that Abel's had been accepted and his rejected. This is plausible because they both brought their sacrifices on Erev Pesach, according to PdR"E that I cited up top. I also assume that Abel would have slaughtered his animal had he known how to kill.)