Real people
don't think that they are evil. People do what they think is good. Because of
this, it's easy to think that people who think differently than you are bad or
even evil. Don't fall into this trap- especially when writing villains and even
heroes. Don't make either character all bad or all good. Give villains a good
goal (that they go about the wrong way trying to reach) or a corruption
backstory. In a pro-life story, the villain will most likely represent the
abortion mindset. Think about pro-choice arguments.
The
overpopulation argument could easily translate into a villain that is killing a
group of people (children or the poor perhaps) to prevent overpopulation. All
life needs to be protected, so don't limit yourself to unborn children when
another group of people works better for your story. You can use symbolism to
relate any group of people to the unborn. Ex: put them in a dimly-lit safe
house next to a river to symbolize a womb. Here's where you can get really
creative!
For heroes,
make sure you don't make them perfect. Let them struggle with selfishness, or
maybe even make them sympathize with the villain. (Here is where giving the
villain some "good" comes nicely into play!) For our villain that is
afraid of overpopulation, maybe the hero grew up in a house with a lot of kids
and not enough beds and food to go around. The hero knows that killing people
to prevent overpopulation is wrong- but he isn't as convicted as he would like
to be. (Give him a character arc- make him change through the course of the
story.)