I think the thing about the characters in this movie is that people seem to misinterpret is that, this was not meant to be like a "movie" movie. The movie is kind of meant to be a sense of realism and in truth, people are by nature really weird and do really weird things. The characters in this movie were meant to be as realistic as possible in the fact they act like real people and not stars.
Truth be told, the fact they all went to find a girl makes sense if you consider it was Rob (main guy's) idea. No one forced the other people to come, they made their choice just like Rob made his choice to go back into the city. No one is saying its the smartest choice but when someone you love is involved, you'd be surprised what people would do.
Be it a child, a pet, a brother, a friend, a lover, a parent, people are not rational or normal when it comes to love. And while yes I admit that by doing this choice and following this path essentially
Spoiler Below everyone pretty much died
, everyone in the movie felt content with their choice. They even addressed it with the girl Marlena mentioning about going back for someone.
Like I said this movie felt real to me and less popcorn movie fiction type. I think it also created a very important picture: things in movies do not always turn out the same. We have all seen dozens of movies with people doing heroic things and charges to rescue people, even knowing when the odds are against them and we never think they are stupid because we know how important it means to them. But in those movies, everything ends up okay.
Here they showed us that not everything does turn out okay, the movie showed us even the most noble or decent of intentions will not change nor will it diminish the severity of the consequences. These people made their choice, it was a choice they knew full well of the risks and dangers, they knew what was involved, they knew about the monster, and they went anyway.