The Watching(2014)

The protagonist, Choi Mi-Hun, witnesses his parents' deaths on his middle school graduation day, struck by a truck. The shock of the incident left him diagnosed with panic disorder, making even the smallest details of daily life feel like life-threatening threats. He becomes a shut-in, a hikikomori. His only friend is Seon-Ah, who grew up in the same neighborhood and occasionally visits him. Then one day, suspicious people move in next door, and their suspicious behavior piques Mi-Hun's curiosity...

(Source: Namu, translated)