When a zombie apocalypse threatens to destroy all of humanity, as one of the few remaining survivors, you have to be resourceful. We quickly learned that many of the main characters in the 2004 remake of Dawn of the Dead were indeed resourceful, as they were able to customize buses with armor plating and barb wire from leftover materials to escape the mob of hungry undead cannibals. However, as we all know, even good plans don't always end well and in horror films, not everyone makes it out alive.
It appeared that their traveling fortress was working quite well, as survivors took turns fending off zombies with gunfire, explosives and a small chainsaw that could fit through the gap in the armor plating. However, the combination of the bus being attacked, some bumpy terrain and an inexperienced member of the clan (Glen) wielding the chainsaw, causes him to accidentally kill Monica and himself with the weapon. We have seen plenty of chainsaw deaths in horror films, but never quite like this. Usually the death is intentional, but I guess accidents happen even in horror films. Oops.