Chandragupta Maurya with the help of Chanakya decided to overthrow the Nandas because
1
Chanakya had been humiliated by the Nandas
2
the Nandas were low-born
3
the Nandas had accumulated a great deal of wealth by extortion and oppression of the people
4
Chanakya wanted to restore the ideal of Kshatriya rule