"'Did I do it?' You'll have to be more specific."

Massacre Girl, Known Killer

Menace
Creatures you control have wither. (They deal damage to creatures in the form of -1/-1 counters.)
Whenever a creature an opponent controls dies, if its toughness was less than 1, draw a card.

Murders at Karlov Manor

Menace
Creatures you control have wither.
Whenever a creature an opponent controls dies, if its toughness was less than 1, draw a card.

Murders at Karlov Manor Promos

Use the toughness of the creature as it last existed on the battlefield to determine whether or not Massacre Girl's ability triggers.

Wither applies to any damage dealt to creatures by creatures you control. This includes combat damage as well as anything that causes creatures you control to deal noncombat damage, such as Incinerator of the Guilty's reflexive triggered ability or the effect of Hard-Hitting Question.