It makes a kind of sense when you consider the ideas that form the groundwork of them, with the core being to avoid personal responsibility.

To them, unborn children are not humans, merely clumps of tissue, so killing them is acceptable even if it's for mere convenience; this allowing them to support abortion under all circumstances. No personal responsibility.

Meanwhile, the criminal is not a bad person, but rather a victim of the society that the leftist blames for all of their own shortcomings; since the criminal didn't actually choose to do whatever heinous thing for them the death penalty, but was instead forced by society somehow, they are absolved of responsibility.