The story of Fullmetal Alchemist: Brotherhood is a tragic one, but it's not without hope. Its major uplifting themes include forgiveness and finding the strength to move past grief and trauma to face a better future. Edward and Alphonse embodied this theme with their failed attempt at human transmutation, and so did Scar the Ishvalan monk.
Scar lost almost everything, including his beloved big brother, in the bloody Ishval Civil War. He gained the power of alchemy from his brother's research (including the transmutation symbols tattooed onto his arms), but at first, he only used the destructive half of alchemy and refused to use the creative half. This set the stage for Scar's personal arc in a few ways, most of them symbolic.
The Symbolism Of Scar's Alchemy
At first, it seems like a tactical weakness on Scar's part to use only half of the power of his alchemy. Edward Elric's own combat style elegantly fuses his martial arts with constructive alchemy, yet Scar spurns constructive alchemy in favor of the destructive half. He will break down matter at will, but to reshape it is an affront to the Ishvalan god. As a monk, Scar can't stand the thought of altering his god's great creation. Out of a sense of piety, Scar will recognize and break down materials, but never reshape them.
This allows Scar to wreak symbolic vengeance on the state alchemists across Amestris, using their own power against them while doing so on his own terms. Scar blasted apart Comanche the silver alchemist and killed off Shou Tucker and his abomination chimera around the same time. Scar even wielded this power against the Elric brothers, then used his incomplete alchemy against the homunculi once he recognized them (especially Envy) as his true enemies. However, Scar was still fighting with one hand tied behind his back, bound to the ways of the Ishvalan people, and it nearly cost him his life.
During the events of the Promised Day, Scar used creative alchemy, praying that his god would forgive him for that transgression. This perfectly symbolized Scar's change of heart, that he identified with more than his own Ishvalan heritage. He was also a hero, fighting to protect the world from monsters and Father's grand plan, and he'd step on a few godly toes if that's what it took to save the day. That act also symbolized Scar's allegiance to the Elric brothers, since he was finally using alchemy the same way they did. He can reshape matter for his own purposes, just like Ed and Al. They're now more alike than ever, and the hate and hostility that divided them was gone.
Other Impacts Of Scar's Alchemy Method
From a strategic point of view, Scar's refusal to use the creative side of alchemy was a weakness. To his credit, Scar was a powerful fighter who could vanquish state alchemists even with this limitation, but he could only get so far. Edward, Al, Roy Mustang and the others figured out a way to counter Scar's fighting style. In the north, Ed and Al chased after Scar in a snowy mining town, finally cornering and capturing him. With constructive alchemy, Scar might have escaped, but he never took that option. Instead, he was bound to a stone wall, where he had an important conversation with the Elrics and Winry Rockbell. Already, Scar was having a change of heart, and this scene pushed him farther along that personal arc. By the Promised Day, Scar had had a full change of heart and took Edward's side, and might never have done that if he'd escaped Ed via constructive alchemy.
Scar's reputation for destruction aided him during his intense duel with Wrath, the sword-slinging homunculus. Wrath was counting on Scar's lack of creative alchemy during the fight, and he was taken by total surprise when Scar finally used the constructive half of alchemy and impaled him on spikes. If Scar had always fought that way, Wrath might have seen that move coming, but in reality, Scar surprised everyone, possibly even himself, with that act. Landing that blow was a turning point in the fight, and it injured Wrath badly enough so Scar got a chance to land the finishing blow not long after. Having survived that fight, with this fresh perspective in mind, Scar was in a position to help foster better Ishval-Amestris relations, promising a better future for all. That wouldn't happen if Wrath had predicted Scar's constructive alchemy and killed him.
