Malls were favored shelters when the first zombies appeared: lots of space and stuff to live on while waiting for rescue. It was a bad idea. When the rescue squads came, malls became traps. Malls are easy to spot and hard to defend, so the whole area turned into a battlefield. Some building collapsed, and barricades went up everywhere. Some blocks even look they got hit by an air strike.
The Toxic Abomination has a special game effect: all standard Zombies standing in its Zone can become Toxic Zombies.