Vengeance Can Wait by Masanori Tominaga

6.20

Hidenori and Nanase have lived together for ten years in a tense, platonic relationship. The wheels of change are set in motion when married couple Takao and Azusa move in next door. After Hidenori catches Nanase and Takao having an affair, he becomes obsessed with watching Nanase through a peephole, and planning his cruel revenge.