This is the most sinful act done by anyone in Mahabharata massacre of innocent people and wild animals in Khandava forest just to create a Indraprastha.
Khandava forest was an ancient forest referred in the epic Mahabharata. It is situated in the west of Yamuna River, in the territory of modern day Delhi. Pandavas cleared this forest to construct their capital city named as Indraprastha. The Naga tribes earlier inhabited the forest. Arjuna and Lord Krishna set up fire to clear the forest. The inhabitants of the forest lost their home. Thus enmity between Naga Takshaka and Kuru kings started which was prevalent for generations.
There is a beautiful story in Mahabharata regarding the burn of Khandava forest. Once Arjuna, the third Pandava and his friend Vasudeva Krishna were sitting in the bank of river Yamuna while Agni appeared before them and said that he was voracious Brahmin who has become tired of eating Ghee from the Yajnas and asked Arjuna and Krishna to arrange something for him to eat. Arjuna asked him what kind of food he preferred. Agni replied that he desired to eat the forest of Khandava. As Agni became pale and sick after having the clarified butter of sacrifice for the continuous twelve years from the Yajnas performed by King Swetaki, he tried to burn the forest of Khandava for seven times. Each time he was failure as the king of Gods Indra protected the forest by himself because there lived Indra`s friend Takshaka Naga, whom he wanted to save. So Agni asked the great warriors Arjuna and Krishna to set the conflagration for him. Arjuna and Krishna did so.
All the animals and forest dwellers were frightened by the fire and tried to escape in any direction from the Khandava forest. But Krishna and Arjuna were ready with their weapons to smite the animals down. All animals died and fell in the Agni`s mouth. They came to know that Takshaka has escaped towards Kurukshetra before the fire was set. Indra tried to protect the Khandava forest but he was defeated in the battle. After the fire has been set up a demon named Maya sought the shelter of Arjuna and thus was saved. The forest burned for ten days. Only six of the forest dwellers were able to save their lives after the fire had extinguished, they were Aswasena, Maya and four birds named Sarngakas. Thus Agni was satiated after eating the Khandava forest.