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Sometime on Saturday, Marlon Haywood received a text from his tenant: “I am on the run from the police BIG TIME.”

In the home they shared outside Pittsburgh, Mr. Haywood knew his tenant, Matthew Karelefsky, as a troubled man. The landlord hid his own guns, fearing that Mr. Karelefsky would hurt himself or someone else.

And in Brooklyn, Rabbi Jonathan Max knew Mr. Karelefsky as a man who had sworn a vendetta to kill him.

The police in New York arrested Mr. Karelefsky on Saturday on two counts of attempted murder and one count of arson. He is accused of setting fire to Rabbi Max’s multifamily home in Midwood early Thursday morning, a blaze that hospitalized at least 13 people.