That kind of exploration definitely requires player buy in from the beginning. That is not something that is okay to spring on players as a surprise. You need consent from players to ask them to grapple with the fallout from an unintentional mass murder. I am surprised it is even a discussion given how quick folks are to throw up yellow cards about other play elements.In a 2 hour action flick? Rarely do you see them address the full impacts of the heroes action. In an 8 hour miniseries or TV series? You often do. Ramifications take time to explore, so in storytelling that has time, you see them explore the impacts of their actions far more often. Comics are full of villains that hate heroes because of the damage the heroes did while protecting the world. Even the MCU movies are finding time to explore fallout - Zemo in the MCU is a byproduct of the devastation in the Ultron fight, for example.
D&D is a long story game. You have time to explore. Dealing with the fallout of your decisions isn't a 'juvenile DM' approach. If handled well, it can be very moving storytelling. I tend to think of young and inexperienced DMs tending towards hack and slash with no story in mind, which would be the opposite of what you attribute to them.
As to "real world" good vs evil: this seems like absolutely not the place to discuss such a fraught, nuanced subject.