Audino, labeled The Kind Nurse, are one of the guests, who are trapped in the Unova Mansion. She's revealed to be the killer at the beginning of Thump!
The reasoning behind her murders, as is revealed in the aforementioned chapter, are the same as the background story she gave in the earlier chapters. Her boyfriend, Sawk, died in an accident and she was unable to save him. This spurred her on to becoming a nurse and eventually finding the motivation to experiment on others (as in she would attempt to kill each one of them and if she was not able to save them, the experiment was dubbed a failure).
Audino's killing blow was a gunshot wound delivered by Krookodile in Thump.
- There are clues throughout the series, that you could've figure it out that Audino is the killer.