Killing what kind of animal, exactly? If the abortion happens when the embryo is a single cell (that may not technically be an abortion, but humour me) does that make it a bit like killing a bacterium?It may not be like killing a human because we have awareness of our existence but its like killing an animal, just because you don't want to live with it.
Do you think abortion becomes progressively more wrong as the embryo/foetus grows in the same way that (perhaps) killing animals becomes progressively more wrong as we consider more and more complex animals? Worse to kill a chimp than a spider. Worse to kill a spider than a bacterium. That kind of thing.
-- Updated Thu Sep 21, 2017 11:11 pm to add the following --
I regularly kill bacteria because I don't want to live with them. Spiders, I tend to throw out of the back door. But if somebody else decided to kill them, I don't think I'd pass judgement much.