why  the titan   arum smell so bad?

Pollak explained that dung beetles, flesh flies and other carnivorous insects are the primary pollinators of this type of flower. These insects typically eat dead flesh. The smell and the dark burgundy color of corpse flower are meant to imitate a dead animal to attract these insects. 

 The bloom uses the scent of rotting flesh to attract pollinators, mainly flies. The first evening of bloom is when the female flowers are open and receptive to pollen. This is the time when the bloom smells the most so it can lure in flies which hopefully have visited another Titan Arum bloom recently and are carrying pollen. The tall center part of the bloom, the spadix, actually heats up the first night to help disperse the odor far distances. The temperature of the spadix will reach about 98F, about the same temperature as the human body. 

The titan arum, therst of two flowers in this list nicknamed the corpse flower, carries the unfortunate designation of being "the worst smelling flower in the world." It smells like — you guessed it — a stinking, rotting corpse. It does its job well enough, since its principal pollinators are flies and beetles that prefer to lay their eggs in dead things. The flower is also clearly titanic, with the largest unbranched inflorescence in the world. This is one massive, smelly bloom. Its vase-like exterior contains thousands of flowers inside, all puffing their fetor into the air. The inside of the spathe is the color of red meat, for added effect. The only good news is that the flower's bloom doesn't last very long, only about 24 to 48 hours, after blooming only once every four to six years.