It honestly depends on how you are writing the web series. How this character dies should also bring about a whole plot line.
Nuclear radiation - Why? What accident happened? What changes in safety regulations?
Sci-fi disease - Why? Was it because of first contact? Does that hinder the relationship with that alien creature?
alien creature - Why? Are they just big baddies who don't have any motivations of their own except to be a big bad or is this an alien creature mob boss and they had to kill this person to keep a secret safe?
Lots of places you can take this death but overall it should reflect what you are going to be writing.