Every death could have been avoided for this series, but I'll just list them here:
Minichen123 - I'm assuming she was fighting a creeper in front of her, and not behind, but don't be fighting enemies in one of the weakest armors. At least go with someone if you're planning on fighting enemies in the wilderness
Holy_Song - Pretty sure she fell off a cliff. That could have been avoided simply by turning around. Never underestimate fall damage
epicskylizard - I think that a skeleton shot him in, but he died a bit after he started being shot. DONT fight near lava, and if you are, run if there are skeletons
BoringOfficeGuy - Do I even need to say? He committed suicide.
While, yes, everything could have been avoided by barricading themselves or staying in a shelter, this was mostly a simple idea.