but I guess that’s always been a foundation of mankind

The above is much more accurate and I believe modern Christianity and religion in general could help turn more people AWAY from such things to be honest but that's a much deeper debate ha.

Christianity is also about forgiveness and repentance, which is a big part of the story too. Joel is living a bad life before he meets Ellie and she presents him with a path to redemption, partly in the form of a cure for mankind. He is hesitant at first but sees the light. In Boston he has abandoned his faith...she helped him find it again in my view.

While he wasn't fully onboard with the cure/vaccine business (the science) he eventually came around to it as he trusted Ellie but when he realised that it would result in the death of an innocent he chose life.

In short he did bad to do good which certainly is a foundation of Christianity.