This is just an incredibly fun satire for genre fans, and I'm a bit surprised it didn't generate larger waves. It uses the old (but not yet tired) trope of an outbreak among people trapped in a building to put a hilariously violent spin on the kind of themes Mike Judge already explored to great comedic effect in 'Office Space' (1999): what happens when a repressed, disillusioned office clerk suddenly has the liberating epiphany that he doesn't give a fû*k anymore and decides he'll stick it to his boss no matter the consequences.
Only that in this version, everyone around said clerk - from the lowest employee to the highest executive - throws their (few remaining) moral restraints over board at the same time as he does. Not through their own will, mind, but because everyone in the building is infected with a virus that makes people act out their most aggressive impulses. As you can imagine, the result of that is not peace, love and happiness - but rather... what's the word... Oh yes: bloody MAYHEM. Needless to say, I had a blast. 7 Stars out of 10.
In case you're interested in more underrated low-budget films and fun B-movies, here's a list with some of my favorites: imdb.com/list/ls054808375/