On my server, the #1 overall cause of death, excluding suicides, is gorefasts but, if you consider that the avg pub team doesn't even make it to fp waves, that number is fairly skewed. On the chance they do make it to waves 7-10, then fleshpounds are the #1 killers followed by: gorefasts for waves 7 & 8, and scrakes for waves 9 & 10.
That's still kind of unfair.
A singular gorefast isn't the cause of death. It gets (sorry for using pokemon meta lingo) a revenge kill on people because of its fast speed and high damage. If you get overwhelmed, the gorefast sprints at you. It jumps around and gets into whatever space is left to reach the you.
On Hell on Earth, a gorefast will do 50 damage on full swing if the player is armor-less. When someone is hurting (<50 health) and running away from a horde that just styled all over their balls, the gorefast just charges in for the kill.
A gorefast has meh health and a lenient, yet sometimes off, hitbox. If
you were good at this game you should be perfectly capable of taking out a single gorefast. If there's too many zeds,
you died because you got overwhelmed, not because the gorefast is OP.
TL;DR: Gorefasts will
kill steal from other zeds because of its high damage. It'll also
finish off people who are hurting because of the fast movement speed when charging. As I said in my wall of text, the gorefast wasn't solely responsible for your death;
you died because you got overwhelmed. This is prefect because gorefast is a squishy zed, meaning it can't take very many hits. It's a threat, but can be dispatched easily, making it balanced.