The unhinged homeless woman accused of stabbing a tourist at Macy’s Herald Square had bought the knife she used in the store — and had once threatened to murder Sen. Elizabeth Warren, prosecutors revealed at her arraignment Friday.

Kerri Aherne was arrested and charged with attempted murder for the alleged attack at Macy’s during the busy holiday season. James Keivom/Pool/ New York Post

Kerri Aherne, 43, was hit with charges of attempted murder, assault, and weapons possession in Manhattan Criminal Court for the savage attack that left a 38-year-old California mother riddled with stab wounds Thursday.

“She had purchased a knife at Macy’s and looked for someone to kill because voices in her head told her that she had to kill someone or else she would be killed,” Manhattan Assistant Di

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