Luke Skywalker’s characterization in The Last Jedi, and its interpretation of Luke, contradict what balance truly means, as defined by the previous two trilogies.
The Force is often simplified into a "light" versus "dark" binary, with merits to both sides and drawbacks to leaning too heavily on one. This interpretation makes sense in video games; however, it is incompatible with the lore established in the films and most non-movie material. Balance isn’t found in Star Wars between the light side and the dark side, because the "light side" is simply the Force, with the dark side being a perversion of it.
During Rey’s first lesson under Luke’s tutelage in The Last Jedi, the latter gives her a surprisingly accurate explanation of the Force. The Force exists throughout nature and doesn’t belong to the Jedi any more than it does to anyone - or anything - else. Luke then voices the shallow misinterpretation of the Force, directly describing balance as “powerful light, powerful darkness.” Considering that Luke was a Jedi Master when he cut himself off from the Force, his lack of understanding of balance is perplexingly dissonant from the Force as it was depicted in the original and prequel trilogies.
The dark side in Star Wars isn’t a natural aspect of the Force, it’s a corruption of it. Throughout the saga and non-movie material in canon and Legends, the dark side overturns the Force’s nature, granting its s incredible power without proper discipline and twisting them into the worst possible versions of themselves. The dark side is essentially a Faustian bargain, requiring beings to sacrifice their values and loved ones and leaving them with nothing but themselves in the end. There are no merits to using a mind-altering power that turns its s into homicidal fascists.
The “light side” is simply a nickname for the Force in its entirety, which already represents balance and harmony with nature. The many failings of Star Wars' old Jedi Order are often attributed incorrectly to the “light side” of the Force. The Jedi may have created a hypocritical and overly dogmatic organization, but the franchise has shown what a balanced Jedi Order looks like. Luke Skywalker’s New Jedi Order in the Legends universe never lost touch with balance in the Force, nor did it deny its a normal life, making the dark side seem far less enticing.
Most of Luke’s behavior in The Last Jedi is irreconcilable with his original trilogy characterization, and his misunderstanding of balance in the Force is no exception. In Luke Skywalker pulled his father from the dark side through his balanced use of emotion and attachment. At no point did Vader act altruistically while using the dark side. Given his personal experience with the merits of the Force in balance and the intrinsically putrefying effects of the dark side, Luke’s lack of understanding of balance in the Force in Star Wars: The Last Jedi is all the more bafflingly out of touch with the prior movies.
