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The more Kelvin uses The Dark Side, the more it horrifies him, until he can no longer use it for the most vital of purposes.
Alternatively: Black Knight Bryce uses white magic for selfish ends, not realizing that he's slowly becoming less ruthless and more compassionate.
Subverted: Kelvin uses the Crimson Deathblade to defeat the Black Knight, then returns it to its rightful place and goes on with his normal life.
...Until he uses the sword in another important fight. Then another. Then another until he's in full-on Blood Knight mode.
Alternatively: Kelvin only used the Crimson Deathblade the once... But it doesn't matter. The more he goes on, the more he's willing to go to extremes. The sword was just an object; it was the willingness to use it that was the gateway to corruption.
Parodied: When Kelvin draws the Crimson Deathblade, it starts the "Dark force" background music. When he uses it to kill, a mailman delivers him a greeting card from The Evil Side. Kelvin wonders if it means anything.
Zig Zagged: Sometimes are means which matter, other times are the ends. Other time is just angst and after a while no one remembers that the protagonist ever did something bad.
Conversed: "Remember in the legends the Blood Warrior of Hellesbe wasn't born that way. He picked up the Black Baldrick and could never see the evil in what he was doing after that."
Deconstructed: Kelvin knows the perils of tempting his own dark side, but tries to rationalize it considering the needs. In the end he falls for the dark side, but is still considered a hero for sacrificing his morality for the sake of others.
Reconstructed: But then as a force of evil Kelvin causes more harm than he originally prevented.
Played For Laughs: "Can you please put down that sword? Every time you pick it up, your power aura switches to ominous red, and it's annoying"
Played For Drama: Kelvin's initial rationale for using the Crimson Deathblade is that it's the only way he can save his beloved fiancée, Indira the Innocent. Its corrupting effects, however, lead him into greater and greater acts of evil, until he ends up cutting down Indira herself in cold blood, leading to a My God, What Have I Done? realization that drives him to suicide.