Basic Trope: A character is heroic but resorts to extreme means to get positive results for the greater good.
Straight: Bob is an Ideal Hero. He never hurts anyone. His teammate Alice is a Pragmatic Hero. Pray so that no one will ever get in the way of the greater good, even by carelessness, because she will do what she has to do! However, when Charlie, the Unscrupulous Hero, kicks the dog without a good reason, she gives him a "The Reason You Suck" Speech that makes Hannibal jealous, out of pure disapprobation, and she never indulges in this. When she discovers that the next generation contains an Apocalypse Maiden who will perhaps free Evulvzus, she designs saving plans, and prepares terrible methods in case it would fail.
Alice is a Well-Intentioned Extremist or a Knight Templar. Believing that someone must be an Apocalypse Maiden in the next generation, and having enough time and possibilities to reform her, she nevertheless gets tempted by the idea of an Herode Plot, almost kills Bob who tries to prevent her from putting it in motion, and enlists Charlie to help. Her sister Claire is one of the targets.
Downplayed: Alice's questionable acts do not really harm anyone directly.
Justified: People are in danger everywhere because of a curse menacing everyone in the kingdom, by prompting the release of an Eldritch Abomination. Despite her love for her sister, Alice feels that her Herode Plot is the only way to save the whole country and BobThe Generous's love for her sister must not stop her.
Alice tries to be this, but can't succeed, because she does not allow herself to use such methods.
She loves her sister too much to do this.
Alice uses these methods and delivers explications to justify neutral behaviors.
Finally, she realizes she has to do it.
Her sister makes her understand that surviving and doing this prophecy's act contents would be a Fate Worse than Death.
Finally, she understands why she should be this kind of hero instead of manipulating them.
Parodied: The work is a parody of the TV tropes's Sliding Scale. Alice, a parody of type 3 Anti-Hero, is so dangerous, and Bob, a parody from Knight in Shining Armor, is so ineffectual that Charlie becomes an unlikely successor for them both. Dallia, who is a Knight in Sour Armor, only lampshades this snarkily before letting Charlie resolve the situation with less damages than Alice thought she would.
We have to do what we have to do! You'll need to prepare the Death Ray, the Person of Mass Destruction, the One-Man Army and the Death Glare. Yes, this one is important too! If it wasn't, I wouldn't be bothering to write it ! Each and every word was needed, and I've got only a few characters left before I exceed my allowed carbon print. Go check Pragmatic Hero for more information.