Obviously, radiation killing everybody isn’t a good thing for the characters, but it has served as a grand unifying threat to forge unlikely alliances between some of the clans.
For the first time in the history of The 100, the big threat of the season is not a person or a group or even a war.
Nobody will survive unless they work together, and this season has been more cohesive because of it.
In Season 4, the Sky People and Grounders alike are facing extinction due to the wave of radiation coming from all the century-old nuclear reactors that are melting down.