It is unusual for a 100-foot tall Bellsprout, known best for eating Butterfree and Heracross whole, to become a symbol of peace and starve itself to death. It is equally unusual for a great conqueror to follow his example, withdraw his armies from conquered lands, and build an enormous tower to symbolize the hope for peace in the world. Violet City has been a very unusual place at some times in its history.
The Bellsprout was a civic fixture, much in the way a skyscraper or a particularly storied tree might be today. It had lived for a hundred years, growing taller and taller; this rare phenomenon was suspected to be because of everstone residue in the soil. It had defended the city from an Alph-led alliance on one occasion, and the general who led it and the other pokemon in its defense soon made himself king. As ruler, he gave the rival city of Alph a brutal sack from which it never recovered, and carved out an empire from Ecruteak to New Bark.
He was also a man who could communicate with pokemon. He noticed after returning from one campaign that the Bellsprout had stopped eating, and asked him why. They discussed philosophy, faith, and pacifism for hours, but the Bellsprout would not be swayed into killing to eat again. Later that night, the Bellsprout died. Both convinced and broken by the pokemon's conviction, the king withdrew from his empire, declared the Bellsprout a god, and erected Sprout Tower in its honor.
To this day, pacifist monks train and meditate there, often scrawny from a diet which does not allow them to eat pokemon. Each one of them remembers this tale and pays reverence to the Bellsprout's spirit in many ways, among them by training Bellsprout of their own.