SB 10.6.10

tasmin stanaṁ durjara-vīryam ulbaṇaṁ
 ghorāṅkam ādāya śiśor dadāv atha
gāḍhaṁ karābhyāṁ bhagavān prapīḍya tat-
 prāṇaiḥ samaṁ roṣa-samanvito ’pibat
tasmin — in that very spot; stanam — the breast; durjara-vīryam — a very powerful weapon mixed with poison; ulbaṇam — which was fierce; ghorā — the most ferocious Pūtanā; aṅkam — on her lap; ādāya — placing; śiśoḥ — in the mouth of the child; dadau — pushed; atha — thereupon; gāḍham — very hard; karābhyām — with both hands; bhagavān — the Supreme Personality of Godhead; prapīḍya — giving her great pain; tat-prāṇaiḥ — her life; samam — along with; roṣa-samanvitaḥ — being very angry at her; apibat — sucked the breast.
On that very spot, the fiercely dangerous Rākṣasī took Kṛṣṇa on her lap and pushed her breast into His mouth. The nipple of her breast was smeared with a dangerous, immediately effective poison, but the Supreme Personality of Godhead, Kṛṣṇa, becoming very angry at her, took hold of her breast, squeezed it very hard with both hands, and sucked out both the poison and her life.

Lord Kṛṣṇa was not angry at Pūtanā for His own sake. Rather, He was angry because the Rākṣasī had killed so many small children in Vrajabhūmi. Therefore He decided that she should be punished by having to forfeit her life.