A retired construction worker who fatally shot five neighbors in his Chicago apartment building left a series of chilling notes beforehand threatening to show “no mercy,” according to prosecutors.

Krysztof Marek, 66, is charged with five counts of murder for shooting four neighbors as they sat for dinner Saturday — then chasing down and killing a woman in another apartment, prosecutors said in Cook County Circuit Court on Monday.

Police then found “multiple cryptic letters” in his condo that addressed “issues with his neighbors” — as well as ones with apparent direct threats taped to the inside of his front door.

“Tomorrow, no mercy without any stupid hesitation,” one of the notes read in Polish, prosecutors told the hearing. “Remember who you are. Remember what this piece of s— is doing to you. Enough.”

Another note also said “no mercy,” adding, “Remember whatever s— they do to you, you control it yourself, not them. Enough. They have to pay for it.”

Judge John Fitzgerald Lyke Jr. said, “What I just heard is evil on steroids.”

Marek is accused of killing Kostadinova Tsvetanka, 43, her husband, Ivaylo Popov, 43, as well as Popov’s mother, Iskra Pourel-Popova, 65, and her partner, David Hanik, 61. Police found all four on the floor around the dining table, their food untouched, according to the Chicago Tribune.

The woman who was shot in the head as she tried to flee was identified as 53-year-old Jolanta Topolska.

After his rampage, Marek put his .40-caliber handgun on a coffee table in his apartment, walked outside and told police, “I think you’re looking for me. I did it,” the court was told.

He had previously confronted his neighbors over petty disputes — telling off a woman for walking too loudly on the floor above his apartment, police say. The condominium association was also seeking to evict him after he failed to pay his mortgage and other fees.

Marek faces five counts of first-degree murder and was ordered held without bail.

With Post wires