By him the waters of the Yamuna, flowing to the ocean, have been poisoned, and the thirsty cows and cow-herds cannot satisfy their thirst. I must slay this serpent, so that the inhabitants of Vraja may live here happily freed from fear. I have descended upon the land of mortals to chastise the wicked, led astray into vicious paths; I shall therefore climb up the neighbouring Kadamba tree and jump down into the pool".

In plain words: Krishna thinks: this serpent has poisoned the Yamuna river, so the thirsty cows and herders cannot drink. He decides to defeat it so the people of Vraja can live happily without fear. He says he came to earth to punish the wicked, so he will climb a Kadamba tree and jump into the pool.