What happens at the end?
You are a parasite that develops inside of a host. However, all beings covet life. But if you choose to live you kill your host.
You are given a sort of option at the 'final level' of the game; whether or not to escape from your host. If you break out of your host, your 'brother', you kill him and can 'live'. If you do not, you die instead, or are permanently stuck inside of him.
At least that's my interpretation. I've played this before anyway.