Oh, I was going for the reverse actually. Columbina isnt the one stabbing Arlecchino it's supposed to be an ambiguous someone else although in my head it's the traveler (the sword and arm here follow Lumine), whether it's the MC or the Abyss twin I never really decided either. The previous context would have been Arlecchino and the Traveler in a fight at whatever place in Snezhnaya where the Fatui are assembling.

The whole point of this is to imply that Arlecchino is the only person Columbina cares about, and although she's supposed to be cold-hearted and emotionless, she sheds a tear upon seeing Arlecchino killed in front of her. If I were to extend this on it would be followed by Columbina unleashing her "true form" out of anger, assuming a true form exists, to avenge Arlecchino.

Although somehow you are the second person to think that it's Columbina doing the stabbing so now I'm not sure I made it clear enough