With his apathetic swagger, cynical outlook on life, and often selfish perspective throughout most of Buffy the Vampire Slayer, Spike has often been viewed as a lone wolf only interested in ensuring his own survival. For half the series, he strategically aligned himself with whichever Big Bad would help him take down the Slayer, and let go of dead weight — like Harmony — that didn't help him realize his goals.
After a serious change of soul, his executive function focused on altruistic purposes, and wanting to survive turned into wanting to thrive; not simply for himself, but for the ones he cared about. Whether he was interested in looking out for himself or looking out for others, certain decisions Spike made were astute and showed the wisdom of his years.
Forming A Brood With Angelus, Darla, and Drusilla
Spike didn't start out a sauntering bad boy right away, and his first few days as a vampire were disorienting. He still carried a lot of vestiges from his time as the mortal William the Bloody, and his temperament was meek, cowardly, and highly gullible. Forming the Whirlwind with Angelus, Darla, and Drusilla was, in many ways, the smartest thing he could have done to survive.
All four members of the group were from the Master's bloodline, and incredibly powerful as a unit, but individually they were able to spite the constraints of their mortal past. As a vampire in the Whirlwind, Spike could destroy anyone who might have ridiculed him as a human, taking back his agency in the process.
Leaving Harmony
Harmony and Spike were not a power couple, nor were they relationship goals, since their entire connection was based on physicality. They had nothing in common, their personalities were drastically different, and they engaged in petty passive-aggressive warfare against one another.
At the end of the day, they didn't treat each other well, and continuing to be a couple would only hinder both of them. Spike was never going to succeed in his plans for Sunnydale with Harm around, and she was never going to get the boyfriend she deserved trying to placate him. Spike breaking things off allowed them both to move on with their unlives.
Fighting Alongside The Scoobies
Whether it was begrudgingly teaming up with Buffy to prevent Angelus and Darla from sucking Sunnydale into hell, taking on Adam and his minions, or fighting off Caleb and his followers, Spike added his considerable strength and prowess as a vampire to their confrontations with Big Bads throughout the series. As Spike transformed into a more heroic figure over the years, he became a dependable ally.
Along with members who had supernatural gifts like Willow or Anya, Spike could use his heightened senses and combat skills to level the playing field. Having a vampire on their side helped the Scooby Gang take on more formidable opponents.
Protecting The Key From Glory
Spike was the only person Buffy could trust who was strong enough to protect Dawn from Glory, but he could have sold her out and betrayed the Slayer if he'd been feeling vengeful. Their relationship had been fractious at that point, especially given the presence of the Buffybot, but Spike put his own feelings aside.
Even after being tortured by Glory and her minions, Spike made an active choice not to reveal the Key's location, even if it meant death. This decision earned Buffy's trust and gratitude and changed her opinion of him.
Falling In Love With Buffy
It wasn't convenient nor appropriate that Spike fell in love with Buffy, but as he told her, "Great love is wild and passionate and dangerous. It burns and consumes." Spike was enabled by his love for the Slayer, even if it caused a lot of friction between the former enemies.
Pining for Buffy made Spike want to change and be worthy of her affection, even if it was a long time in being returned. It ultimately led to so many of the good moments in his unlife, including being the "good guy."
Ditching Dru
A vampire's bond with their sire is incredibly strong, and Spike remained loyal to his despite how it affected him mentally and physically. Drusilla herself was the product of Angelus's sadistic manipulations, and she was in no condition to shape a young vampire mind of her own.
Despite having dalliances with Angelus and hurting Spike's feelings, Drusilla would reappear in his life only when she wanted to prove to herself she could still hold power over him. While some fans thought Spike and Drusilla were a better couple than him and Buffy, getting Dru out of his life was one of the best things he could do for himself, and for the citizens of Sunnydale, who would always be their next victims if they decided to go on a rampage.
Giving Buffy & The Scoobies Insider Knowledge
Whether it was how to defeat Dracula, the rules of kitten poker, or breaking down exactly how he dispatched two slayers, Spike always came through with the 411 on vampires, demons, and Big Bads that were preparing to unleash the forces of darkness on Sunnydale. Given he was over one hundred years old, deciding to give the Slayer and her Scoobies access to his many decades of experience with the supernatural was invaluable.
Time spent in the library or the Magic Box with Giles pouring over tomes was only so helpful, and having a vampire with intimate knowledge of the underbelly of the underworld allowed for Buffy and the rest of the Scoobies to apply real solutions in the field.
Being There For Buffy When Her Friends Weren't
After Buffy was resurrected, she went through a period of confusion and discomfort. Feeling as though she'd been wrenched from a sort of utopian peace by her friends, she struggled to reintegrate into normal life in Sunnydale. Even going on patrol was difficult for her, and being around her friends after what they'd done was even harder. The only person she felt comfortable around was Spike.
As he so memorably sang to her in "Once More With Feeling," "The pain that you feel can only be healed by living. You have to go on living." Adjusting to life after spending time in what came after it meant that Buffy and Spike shared a bond that was unlike anything her friends could comprehend. One of the worst things the Scooby Gang did was alienate Buffy, but Spike was there for her in ways they couldn't be.
Going To Get His Soul
After Spike violated Buffy's trust in one of the most shameless ways possible, he decided he would try to regain his soul, joining the ranks of Angelus and other re-souled vampires burdened with the guilt of their numerous victims. Spike needed to be able to prove to Buffy that he'd changed and was capable of being honorable.
His quest might have caused him to temporarily lose his grip on reality, but it was worth it in the end. Buffy didn't understand his choice when they reunited, but its effects couldn't be denied; he was a more compassionate, selfless person around the Scooby Gang and the Potentials, committed to being on the side of the righteous.
Sacrificing Himself For Sunnydale
Armed with an amulet, a recently restored soul, and his courage, Spike decided to make the ultimate sacrifice so that Buffy, the Scoobies, and the rest of Sunnydale could survive. The series closed out on a once selfish vampire punk being the bravest of them all, knowing he would only have one chance to truly be the hero and step into the light.
Spike loved Buffy and wanted her to go on to live a full life, surrounded by those she cared the most about. The decision demonstrated an incredible amount of compassion and altruism in a character who, in previous years, would have been rooting for the Slayer's destruction and Sunnydale's annihilation.