SB 10.50.9

etad-artho ’vatāro ’yaṁ
 bhū-bhāra-haraṇāya me
saṁrakṣaṇāya sādhūnāṁ
 kṛto ’nyeṣāṁ vadhāya ca
Word for word: 
etat — for this; arthaḥ — purpose; avatāraḥ — descent; ayam — this; bhū — of the earth; bhāra — the burden; haraṇāya — for removing; me — by Me; saṁrakṣaṇāya — for the complete protection; sādhūnām — of the saintly; kṛtaḥ — done; anyeṣām — of others (the nonsaintly); vadhāya — for killing; ca — and.
Translation: 
This is the purpose of My present incarnation — to relieve the earth of its burden, protect the pious and kill the impious.