Portal 2, however, really broke it down for me. See, in the game you reactivate the evil AI System GLaDOS. Even though she's a computer, you just can't help but have animosity towards her. She's trapped you in an endless maze of test centers, with deadly traps and puzzles -- and when you reactivate her, she's even more ticked off at you because you killed her in the last game - so the feeling is mutual. You simply hate GLaDOS. Yes, she's ultimately a piece of software - but she's suppose to be a sentient piece of software, right? Either way, she's trying to kill you - so you're understandably upset with her.
In the course of the game a new character, a personality sphere named Wheatley, helps you defeat GLaDOS once again, by installing himself as the new AI of the facility. Almost immediately Wheatley is drunk on power and betrays you. You're immediately sent off for more testing. Eventually you run into GLaDOS again - discovering that Wheatley put her in a potato powered battery. Wheatley is just not equipped to run the facility, as it's clear the place is going to blow up. So you reluctantly team up with Potato/GLaDOS. You immediately think "I don't care if we have a common enemy, you're just as worse as the current guy." But through the course of your adventure through the bowels of the Aperture Science facilities, you discover the origins of GLaDOS. Her personality was originally based on the wife of Aperture Science's Founder, Cave Johnson. Cave Johnson wasn't going to live long enough for technology to be able to download his mind into a computer. So he made the scientists promise to preserve his wife's mind. Her name was Caroline, and her mind was the basis for what ultimately became GLaDOS. You begin to feel some sympathy for GLaDOS. With the rediscovery of her origins, GLaDOS even becomes a more supportive partner in defeating Wheatley.
What really began to show the inner working of GLaDOS, and what made her so evil to begin with, was seeing all the same forces that effected GLaDOS now effecting Wheatley. GLaDOS was programmed to always want to test. In fact, there's somewhat of a drug-like high she gets when testing. Wheatley is experiencing the exact same effect - as he struggles to put you through more and more puzzles. Apparently there begins to be an immunity to the High Wheatley is experiencing -- GLaDOS had a much more sophisticated (smart) system, and was able to cope with the highs and lows of the effect. Equally, it's shown that GLaDOS cannot give a test subject the answer to a puzzle - or it hurts her.
Essentially GLaDOS' programming we, as the players, so easily dismissed before -- it's all laid bare for us to see. She was truly trapped in her programming - with so many forces prodding her to be the relentless and evil testing machine she is. Being this way is a clear result of GLaDOS' nature. But what about the nurturing side of the equation? Yes, GLaDOS had the basis of Caroline's mind to rely on, but it clearly all went horribly wrong. I doubt the scientists of Aperture Science where very nice (or ethical) in how they programmed and used her either.
For GLaDOS the building blocks are very basic, especially in comparison to real people's personalities. The biggest tie-breaker in the Nature vs Nurture debate is, of course, free will. Even with all the programming and experience working against her - once she's reinstalled as the AI of the facility, does GLaDOS demonstrate free will? She's been on this big adventure with her biggest nemesis - and even learned about her origins as Caroline. You helped her regain her old position. So, do we see some gratitude? Did GLaDOS change? Did GLaDOS learn anything? In fact, she did! Rediscovering Caroline taught her a lot -- which is why she promptly deleted Caroline from her memory. It doesn't look good - GLaDOS is back in her old body, and is quickly returning to her evil ways. Yet -- she makes a decision. She releases the main character, actually bringing her all the way to the surface. She sort of rudely kicks her out, but GLaDOS did indeed keep her end on the bargain. Everything that made GLaDOS what she was, and even with everything she learned deleted, GLaDOS made a different choice than expected.
We real people, of course, make decisions all the time. They are so heavily informed on almost every aspect of our entire being -- but we ultimate use free will as that crucial tie-breaker between Nature vs Nurture. I know it was just a story in the game, but it really struck me as very instructive. GLaDOS went from a hated antagonist, to a sympathetic character you more fully understood, and ultimately now regard her as a more realized character. She's still no doubt going to be evil in the future - but once you see what motivates a person to be like they are - it changes you're perspective on them. We experience it all the time in real life - perhaps we hate a particular person, but later learned more about their troubles. You don't always stop hating said person, but knowing what motivates them to be so dislikable gives you a new perspective. We all have underlying motivations that make us all who we are, and it can be very informative for us to remember that before purely hating or judging someone. I'm not saying it's ever an excuse for someone, using their past and troubles to justify bad behavior - but it's instructive to for others, and maybe even the person in question, to realize what exactly is motivating their behavior.