Arguments for Snape killing Dumbledore for a "good thing":
* Dumbledore's plan: Dumbledore knew he was going to die and planned it out meticulously. He wanted Snape to kill him to ensure that the Horcrux in his body was destroyed and to protect Snape's cover as a Death Eater.
* Snape's loyalty to Dumbledore: Despite his past animosity, Snape was deeply devoted to Dumbledore's cause and was willing to do anything to protect him. Even killing him was a form of loyalty.
* The greater good: Snape's actions, however brutal, were ultimately aimed at defeating Voldemort and protecting Harry Potter. He understood the consequences of Dumbledore's death and believed it was necessary.
Arguments against Snape killing Dumbledore for a "good thing":
* Moral ambiguity: Snape's actions, while driven by a noble cause, were still morally questionable. Taking a life, even if it's part of a larger plan, raises ethical concerns.
* Dumbledore's vulnerability: While Dumbledore planned his death, he was still a vulnerable old man who was being manipulated by Snape. The act of killing him was not entirely consensual.
* Personal motivations: Snape's own history and his feelings for Lily Potter likely played a significant role in his willingness to kill Dumbledore. He was trying to atone for his past mistakes and protect Harry, a surrogate for Lily.
Conclusion:
Whether Snape killed Dumbledore for a "good thing" is a matter of perspective. There are arguments to be made on both sides, and ultimately, it's up to each reader to decide. Snape's actions were complex and driven by a mixture of loyalty, duty, and personal motivations. His actions were undeniably crucial to defeating Voldemort, but they also raise ethical questions about the nature of sacrifice and the cost of war.