I think that humanities biggest weakness is our over confidence in good.
Of course evil can win. Evil certainly won first and second world war and evil have continued to win and rule over humanity on countless of events throughout history.
The weakness lies in good and evil not playing to the same rules.
Good doesn't assassinate their opponents.
We tend to forgive and want to see good in others.
We need to change this. We need to become fanatics if we ever hope to win.