Umm.... that execution scene in Life of Brian comes to mind.
But for something more original...
The year is 2077. The world's supply of reachable oil, coal, and natural gas ran out decades ago. Solar and wind power have proven to be ineffectual. However, this has not had a severe negative effect on the world. After the population stabilized, the vast majority of people are able to live relatively happy, peaceful lives. They simply make do with less technology.
BUT THEN!
Aliens attack! Raining down from the skies in UFOs that look like they were stolen from a 1970s alien flick, invaders declare war on the Earthlings. It looks like all is lost. Our weapons are useless. We have no source of energy. Scientists are able to develop a powerful weapon that could defeat the aliens, but it requires a massive amount of energy.
Just when it looks like extinction is inevitable, a team of scientists miraculously discover a large piece of antimatter, suspended in some sort of sustaining field. However, this anti-matter has a very specific configuration, and all of it is needed to provide enough energy. After careful analysis, the scientists come to the conclusion that a human most closely matches the opposite of the antimatter.
An aged member of the scientific team volunteers to be paired with the antimatter, knowing that it will be his destruction. His sacrifice enables the military to power their weapon, and defeat the invaders. However, one man decides that an alien species is too valuable to exterminate completely, and captures a wounded invader. He lets the Alien roam free in his backyard.
Later that night, the alien escapes and kills the man. It goes on to breed a new army, and eventually takes over Earth. But the Alien remembers the human's kindness, and decides to keep one human as a pet. This human enjoyed a wonderful life, getting to go to hundreds of new worlds and civilizations. He eventually served as an adviser to the alien race, steering them in a more peaceful direction. After many happy years, he chokes on a space peanut and dies.
I WAS THAT MAN.