An 80-year-old was knifed to death inside his Manhattan apartment by a man whom he had recently welcomed into his home when he was homeless, according to police and family.

Lev Vayner was fatally stabbed in his first-floor Washington Heights apartment by his 45-year-old roommate, police said. The suspect was identified as Alon Riabichev, who neighbors confirmed was the man hauled away from the 34th Precinct Monday afternoon after being charged with murder.

Vayner’s nephew said his uncle had invited the 45-year-old Riabichev to stay with him on Thursday after he was kicked out of his home — even though the nephew and Vayner’s daughter both objected to the idea.

“I said that this is a bad idea because you should really make referral to shelter,” the nephew told NBC New York.

Neighbors were shocked to learn what happened to a beloved member of their community, as those who knew him were left wondering why anyone would hurt Vayner.

“Everyone loved him and knew him, and he did good for everyone…he was always cracking jokes and helping people,” said Irina, a neighbor of the victim.

Riabichev had been the topic of discussion among those who lived in the building lately. He allegedly had been kicked out of the apartment he lived in with his family and was camping in the hallway. Irina said Riabichev had grown up in the building and suffered from mental illness.

“The whole building felt bad for this guy because we all knew him and when stuff started going down with his mom, you had to walk pass him every day,” said Moses Waterland, who lives in the building.

A woman who answered the door where neighbors said Riabichev previously lived with family declined to comment.

Attorney information for Riabichev was not immediately available.