1) The narrator feels a sense of being followed and watched since an event that changed his life and made him unhappy.
2) He describes encountering a girl he was interested in, but another man intervened and took her away, deeply hurting him. This caused him to develop a dark and unstable personality.
3) The narrator plans to get revenge against the man who ruined his life and took the girl away. He believes the man knows he is coming for him and will kill him slowly and painfully.