If Lapis kills all the commoners, that means they'll have more bargaining power with regard to fair employment. Heh.
(I don't think Neko Tarou actually thought about that)
Basically it [win-win] for most parties really.
So technically it pretty much a [win-win] for both the commoners and nobles technically. (Technically Lapis wants to kill all the Anti-noble fraction, she probably willing to accept or cause random commoners + low rank nobles scum to die though in the process if she need to for her plan though XD)
The commoner left get treated better since there is less commoner/people= Reason why War can be good for the economy (True but false)...lesser people alive= more people have an chance at better employment since the employer/company cannot just pick other people willing to work for an smaller pay + The fear, effects and result of the recent revolt that was put down probably made them rethink of abusing their workers.
Though the {Anti-noble fraction} are probably b*tching and complaining about being killed but hey if they truly cared about having commoner be treated better/have a better future then they should be happy with the end result that lead to their deaths correct lol?
The nobles now don't have to worry about an revolt since a lot of commoner/Anti-noble fraction is now dead. The commoners now have to live in their after-math of the revolt being defeated, their firepower is now low while the noble side/fraction probably also suffered damage to...basically an almost statement.
Basically it an win-win for everyone except for the Anti-noble fraction + a few innocent commoners that got caught in the crossfire + (Lapis who died...though she probably didn't mind if she got her revenge like in the original game) XD
If you read the previous chapters than Lapis pretty much got her revenge or almost got her revenge (Assuming she killed at least 85% of the Anti-noble fraction before she died) in the original timeline/game.
= The Anti-noble fraction is dead, The game Heroine is now in control of the commoners + she gain a lot of respect to stand beside the prince + The prince gaining people respect for stopping Lapis' rampage while accept his fire magic.
Honestly Lapis does not care about the Commoners being abused or for commoners getting better treatment/pay for their work (Though she does look down on them for their low education and due to the Anti-noble fraction for the death of her adopted sister XD), the commoner having better lives does not make Lapis unhappy or happy...in fact the Game original plot ending could be 70% of Lapis' plans to help out the Prince and better support the kingdom while planning her Revenge.
TL;DR
-- Lapis does not care if commoners get better treatment or if their treatment gets worse, her current goal is to kill the {Anti-noble} fraction for revenge. She would just shrug at both good or bad options for the commoners in my opinion of what I read, it not like commoner getting better treatment will make her angry unless they are part of the {Anti-noble} fraction.
The prince & game heroine gaining respect and kingdom stability is just bonus points before her death XD