This may depend on your theology. I know in the Catholic church there are heirarchies of sins, with some worse than others, and many Protestant denominations assert that all sin is deadly.
All significant sin is deadly, hence the label 'mortal'. That is a theological issue. Abortion is a matter of Natural Law, intrinsically evil regardless of your religion, or lack thereof - just like murder, rape, arson and the like.
I'm pinging the Catholic List to ask those better read than I to explain the difference between Natural Law and religion. I do know that our country was founded on a bedrock of Natural Law.