Usually with anime, there are the heroes (aka protagonists), and the villains (aka antagonists). There might be some characters that switch sides, or the sides might not be ultimate good or evil, but you can almost always draw those lines.
In Aru Zombie Shoujo no Sainan, you think the group of college kids are the protagonists, but you are quickly shown they are not.
When I heard the voices of the 2 girls, I thought "Oh, OK, they're the protagonists", since, in my experience, Japanese voice actors tend to play roles on the same side (good/evil), with similar characteristics, and those were the voices of the good guys. Though I was also quickly shown otherwise.
At the end, I was thoroughly confused as to who was on which side, or if there were even sides at all.