Correct me if I'm wrong, I'm probably remembering it wrong, but wasn't Darkrai portrayed as being evil? Purposefully causing nightmares for children seems pretty evil to me.
There are excuses out there like "it wants to play and/or it's protecting itself and doesn't realize what it's doing is harmful," which seems to be the narrative that the series' writers prefer. It's fifth generation Dex text states: "To protect itself, it afflicts those around it with nightmares. However, it means no harm."
However, I kind of take issue with that narrative given the presentation of the Strange House and the child in Canalave City. In the former, the girl's ghost says: "In the dark dream... I heard my dad’s voice... Forget about the Lunar Wing... Please stay here with me..." She says some more before and after, but that one makes it seem like Darkrai is impersonating her father and trying to entice her to stay in the dream... which would probably kill her. Needless to say, she died as a result of the incident. The kid in Canalave mentions that Darkrai is watching it as he suffers through his own nightmare episode, which is pretty creepy and hints at malice over self-defense.
For that reason, I could see Necrozma being explicitly antagonistic, or even "evil." They've flirted with the idea of an evil Pokemon before, and simply haven't truly committed to the idea, instead inventing an excuse for its behavioral patterns or framing the acts as being done under the orders of a trainer. Necrozma's literal concept has been shown to be eating light, and that's not exactly the most benevolent concept in the world. Sure, it can be made out to be a simple biological function, but it comes with some very dark implications (no pun intended), much like Darkrai's themes.