The theme of the movie is absolutely amazing. What makes a human, human? When empathy is quantified and measured by machines, and its seen as morally okay to shoot down an unarmed Replicant fleeing for its life, because, it's not execution... it's retirement. "More human than a human is our motto" Tyrell says.

It makes us question our own humanity. Roy just wanted to live longer, just like humans want to live longer. Replicants want to love, and live. It makes us reflect on our experiences in our short life and see how it makes us the person we are today.

That's why I love the speech by Roy before he dies. "All these memories... gone, like tears in the rain." The symbolism and thematic elements of that final conflict are just so good.