Actually, only a few of them were demons. The rest were provably humans induldging on some demon enduced sinning.what i want to say is that Dante saw this girl's demonic form in Limbo and killed her. What was seen by the public is Dante killed an ordinary girl. What Dante does in Limbo has an real impact on human world. He is not doing things in void.
see the source:
One witness that escaped the horror told Raptor News that the assailant mercilessly picked off clubbers as they begged for their lives, with one 18 year old girl brutally slain as she knelt praying.
As for the girl and the witness that saw him offing her? They provably don't exist. Just like the other lies they made about him, it's provably all a lie.