Here it says "va'Tdaber" (Asalyah spoke to the descendants of the kingship). In Melachim II, 11:1 it says that she killed them!
'Rashi': "Va'Tdaber" is an expression of Dever 1 (plague). About this generation, David said "la'Menatze'ach Al ha'Sheminis" (Tehilim 12:1). He saw with Ru'ach ha'Kodesh that Asalyah would kill all seed of the kingship in the eighth generation from Shlomo. David prayed that there be a remnant of his seed ? "Hoshi'ah Hashem Ki Gamar Chasid" (12:2). In Melachim, it says (implies) that she killed them via poison. It is written [that Yehosheva 'stole' Yo'ash from ha'Memusim, and we pronounce it ha'Mumasim. We expound both of these. Mumasim implies that they already died. Memusim implies that they are dying via afflictions. This is via Kishuf 2 and poison; they do not die immediately, rather, their illness worsens and they die over time.
Radak: This is like "Dever Kaved Me'od" (Shemos 9:3). I.e. also here it means that she killed them. Malbim ? this explains how she killed them, via a plague and poison.
Why does 'Rashi' mention witchcraft? Perhaps it was poison via witchcraft, e.g. Kasa d'Chershin (Chulin 84). Or, Kishuf refers to astrology; she gave to them a food that she knew via astrology that it will kill one who eats it then. E.g. it is dangerous to drink water at the moment of the Tekufah (YD 116:5). (PF)
