I was curious as to what "door key nippers" might be. Evidently they were some type of lock-picking device. I found this story:

Feb. 16, 1903: "Stealing was going on all over the city, and killing resulted.

"Frank Nerski fittingly closed a career of sin and crime with his arms around his swag — shot by two policemen as he attempted to escape earthly law. The 'key-nipper' man and other burglars robbed at least four separate dwellings and as usual escaped arrest."

Earl Boebert writes:

A key nipper is a thin forceps or needlenose pliers. People would  lock the
old-fashioned "skeleton key" locks from the inside and leave  the key in to
frustrate lock picking. The thief would grab the key  with the nipper from
the outside and turn it to unlock the door. An  example of the security
axiom that for every countermeasure there is  an attack and for every attack
there is a countermeasure. (The countermeasure against key nippers is a
deadbolt lock).

The lack of legitimate use for the tool let them to be
categorized as  burglar tools, whose possession in those days was
illegal.