The 2003 anime version of Fullmetal Alchemist is drastically and dramatically different from the more faithful 2009 manga adaption, Fullmetal Alchemist: Brotherhood. These differences resulted from the manga's ongoing status at the time of the first series, resulting in an anime-original storyline for the latter half of the plot.
One of the biggest differences is the anti-hero Scar and how he grows throughout the respective series. By the end of FMAB, it's clear that Scar has transformed into a man who surpasses his old self in every way. Here's how.
In Fullmetal Alchemist 2003, Scar’s brother attempted to bring back his deceased love using the forbidden art of human transmutation. This act failed, however, and created the homunculus known as Lust which would serve to be the impetus for the brother’s banishment. Later on during the Ishvalan Civil War he sought the Grand Arcanum, an ancient forbidden alchemy that was taught to Scar’s brother by the Ishvalan Exile. By drawing a series of transmutation circles on one’s body, they are able to take the souls of either the deceased or living and use them to create a Philosopher’s Stone inside themselves.
Unlike alchemists who go through three distinct processes to transmute objects, Scar is unique in that he chooses to only use the first two processes. The three processes of alchemy are 1) understanding of the molecular composition, 2) destruction of the material and 3) reconstruction into a new form. Ishvalans are philosophically opposed to the third process, believing it is heretical to make god’s creations better than they already were.
When Scar and his brother try to escape the war, State Alchemist Kimblee intercepts them, giving Scar his eponymous injury and blowing off his right arm. Before dying, his brother attaches his own arm, tattooed with the Grand Arcanum, onto Scar and tells him to continue living. This procedure saved Scar’s life and gifted him with his most dangerous weapon: an arm that can perform the first two processes of alchemic transmutation. With this newfound power of destruction, Scar began a campaign as the “wrath of God” to massacre Amestrian soldiers and State Alchemists by destroying their organs with his right hand.
Scar later manipulates the people of Liore into helping him create a giant transmutation circle throughout their city. In a final battle against Alphonse and Scar at the end of 2003's Fullmetal Alchemist, Kimblee uses the last of his strength to transmute Alphonse into a living bomb while Scar is fatally shot by an Amestrian soldier. Recognizing his imminent demise, Scar uses the last of his power to complete the city-wide transmutation circle he drew and create a Philosopher's Stone within Alphonse to save him. This circle ended up killing hundreds if not thousands of Amestrian soldiers and alchemists, and would serve to horrify the man Scar became in Brotherhood.
In Fullmetal Alchemist: Brotherhood, Scar lost his arm in an almost identical way. State Alchemist Kimblee ambushed a group of Ishvalans and blew them up with his explosive alchemy. Scar was left in shock from blood loss due to losing his right arm in the blast, so his brother attached his own arm to young Scar. This gave him the ability to access the first two processes of alchemy merely by touching it. As Ed notes later in the series, whether Scar chooses to admit it or not, he is a very skilled alchemist and highly intelligent to be able to recognize the materials of what he transmutes at a glance.
Like his 2003 counterpart, Scar went on a quest of vengeance to kill State Alchemists and Amestrian soldiers. An important difference, however, is that he is not trapping their souls anymore in FMAB. This likewise signals a change in character, owing to his more compassionate portrayal. As Scar's quest goes on and he begins killing high-ranking military offers, Ed and Al confront him over his actions. Winry catches up to the three and learns that Scar was the one who killed her parents, leading her to hold him at gunpoint. Ed leaps over Scar in front of Winry, shielding her from his grasp. Reminded of his brother's courage in shielding him when Kimblee attacked, Scar began to have a change of heart over the extremity of his mission.
Scar later continues his journey with Yoki, Marcoh and Mei Chang. This part of his character arc in FMAB is notable for his increasingly relaxed demeanor and decrease in acts of violence against random Amestrian soldiers. After fighting with Ed and Al in the North City, Scar is wounded and captured. He is bandaged by Winry and introduced to Major Miles, a fellow Ishvalan who is trying to change the military's perception of Ishvalans from the inside. Scar is visibly touched by speaking to Miles and begins to truly scrutinize whether his actions are for the greater good.
All of Scar's character growth would pay dividends in his ultimate fight against King Bradley. After stabbing and pinning Scar's right arm to the ground, the King attempts to finish him off by thrusting his sword into Scar as he is lying on his back unarmed. If this were Fullmetal Alchemist 2003's Scar, he would've died right here. However, Brotherhood's version managed to turn the tides by surprising both Bradley and the audience. In the interim leading up to the Promised Day, Scar tattooed his left arm with the symbols of all three alchemic processes.
This allowed Scar to transmute the ground below him into a jagged mass of rocks that served to pierce Bradley's dominant right arm and throw him completely off guard. Instead of relying on the vast network of Philosopher's Stones that ran underground -- as they had unknowingly been doing since Amestria's inception -- Scar, using his knowledge of alkahestry, activates a reversed transmutation circle to allow alchemists to tap into the power of tectonics.
If the two Scars were to ever fight, it would result in Brotherhood's version coming out on top using a lethal combination of his destructive and constructive alchemy. The emotional growth Scar undergoes in FMAB would make him pity the man he was in the 2003 adaptation. Brotherhood's Scar would even compare his old self to Bradley, likely saying they are both using giant human transmutation circles for petty reasons.
Using the wisdom and patience acquired from being an Ishvalan monk, Fullmetal Alchemist: Brotherhood's Scar would not only incapacitate his old self but likely also attempt to reason with him. If words fail, he still has the unique ability to use his left arm to transmute the surroundings of his old self, incapacitating his opponent and his arm, then use his own right arm for the finishing blow.
