A man who killed a gal pal and himself was so obsessed that he staked out her house for three months while she was in China, neighbors said.

David Chea, 55, would sit for hours in front of Lori Eng’s empty 56th Road apartment in Flushing, residents of the tree-lined street recalled.

“He would say, ‘I am watching the house for her,’ ” a neighbor said.

Eng, 51, returned Sunday. Chea visited Wednesday afternoon, and they argued. He shot her twice in the chest at 5:10 p.m., then shot himself in the head.

Both died at the scene.