Although it would make sense for her to be affected by Nihilego's toxins beforehand, I wonder if she was really infected beforehand? I think that the Ultra Recon Squad would have played a difference there. They mentioned the crisis related to Necrozma that would lead to her excuse to find it and capture/defeat it.
I am tempted to make some comparisons, since Nihilego venom dulls your consciousness and makes you act on your most extreme.
1. During your first visit in Aether Paradise, Nihilego shows up in a wormhole. Nothing different happened in either games until after you defeat it, then Lusamine talks to herself about getting something back.
In S/M she flashes an evil smile and talks about protecting/ loving Nihilego. Then starting being vague on what is going on down at the labs, Wicke admitted that it may have not been an accident for the wormhole opening up.
In US/UM, that evil smile does not happen and the URS show up to explain their part in assisting Aether. In contrast, Lusamine is MORE open to what was happening down at the labs, they were doing an experiment that didn't work as intended, Wicke treats it like it was an accident.
2. Next, when you go rescue Lillie. Lusamine in both versions verbally talked down on her, in S/M she had an enraged snarl on her face, showing hate. In US/UM, she had a disappointed/ hurt expression. In S/M, the verbal abuse continues at its worse by saying Lillie wasn't beautiful and told Lillie to stay around and watch her summon the beasts, knowing it might kill Nebby. In US/UM, she called Lillie's behavior unattractive and talks about still wanting to protect her from Necrozma. In S/M, Lusamine taunts Lillie more about forcing Nebby to use its power whether it wants to or not, with more cruel words; in US/UM Lusamine still said that using Nebby's power will kill it, but if it is for the greater good, it will have to be done. Then she leaves to her freezer closet with parting words. S/M it was more cruel taunting with a cruel smile. While in US/UM Lusamine gave Lillie credit where it is due for keeping Cosmog safe and thanking her with a more genuine smile. (To Lusamine, the words are more kind, when they come out... it is basic villain cliche, thanking the hero for doing something for them.)
3. Now for the confrontation in the freezer room. In S/M, Lusamine rambled about how Nihilego must be suffering in Ultra Space and thinks letting it out to do what ever it wants to Alola would help it, while screaming at her kids for stealing her stuff and betraying her. Saying that they no longer matter to her, she even taunts in how many wormholes will open up if she used the whole Cosmog.
In US/UM, she rambles how she can't love those close to her if the light is stolen, taking it upon herself to go fight Necrozma. In her childrens pleas, specifically when Gladion mentions Mohn Lusamine acknowledges the memory of what happened that day as her excuse to go after Necrozma by herself. Despite like in S/M, she called her kids wretches for leaving her, she is still determined to protect them from Necrozma. (Like I said before, Necrozma was her excuse to go into Ultra Space and look for Mohn, the threat this pokemon poses, is something she tells others and herself to mask the more selfish reasons.) With the intended goal of using Nebby to make just one wormhole that she might have control over.
In S/M she used the box without hesitation, taunts Lillie more and use it to open the wormholes with psychotic glee. Only caring about Nihilego, ordering Guzma that she doesn't want to hear from the annoying kids again and wants to silence (Potentially a homicidal metaphor.) you the player. In US/UM she hesitates to use the box and when Nebby resists, she gave praise to Cosmog for its love of Lillie and was rather happy that her kids grew up away from her eyes. But still bound to her self appointed duty, does not want anyone to interfere and challenges you the player to silence any worries her kids might have for her, by proving how she can fare in battle. (This one, is not a homicidal metaphor.) Post battle, she gives a praise to you the player, has Guzma come along with her because she wouldn't be fighting Necrozma alone, then uses the Nebby box with no sign of joy in it.
S/M Lusamine, was acting on her most extreme without the moral inhibitor called a consciousness, while rambling about the Ultra Beasts or specifically, Nihilego. US/UM Lusamine was much more tame while having other priorities on her mind.
In S/M, Lusamine was high on the toxins before hand, given the symptoms. Which gotten worse when she went into Ultra Deep sea and gotten even more of the drug.
While in US/UM, Lusamine was an egotistical loon that was doing things her way, while using the greater good as a counter weight to convince others and herself that it is. As stated, capturing/ defeating Necrozma who was a proven to be a threat when he came, was her excuse to go into Ultra Space and look for Mohn. Only to get her teeth kicked in by Necrozma and endangering the very region she vowed to protect.
