The point of the sign is to show how the city treats the homeless. In the poster homeless are seen as rats that the city is trying to eliminate, this makes the city the villain not the homeless people. Casting it in such a morbid and cruel way highlights just how terrible Chicago treats the homeless (Which may or may not be true. I couldn't tell you because I've never been to Chicago) and is meant to invoke such a grave reaction to try and turn peoples focus inward to themselves, so maybe they realize what they are doing. But in this case it has seemed to go array and people have missed the message the artist was trying to portray and taken it offensively. To be diplomatic about it though I can't speak for the artists intentions, nor the message of their work but that's just how I interpreted the piece as an art major.
