You are a HECU soldier tasked with infiltrating the White Gate Research Center and eliminating everyone associated with illegal Xen research. Spare no one, not a single living being. In this mod you'll be: Murdering scientists and security officers blowing up stuff with c4 shooting people with the autocannon of an APC in a road rage Enough said.
Scientist Slaughterhouse: The Single Player Campaign.
A very unusual mod, in the sense that your enemies encompass Barneys, scientists and a itchysaur only. The mapping is very flawed, with remarkable things such as a spiral staircase you have to crouch-jump in.
I really liked the idea of going in and killing the scientists, like the HECU in original Half-life. I was all giddy as I clawed onto a window in which I saw a whole room of them evil scientists panicking. The best part of the mod is definitly that it isn't all just run & gun; the scientists barricaded themselfe and took to hiding, making the whole experience a puzzle game in which you have to find out how to reach every single one of them.
It is unfortunetly very flawed. Worth a go though for the unique experience.