Well, often there's the theme of a corrupted Buffy. Maybe she's been influenced by dark forces or has made a deal that turns her evil. And then there's the theme of how this affects the entire Scooby Gang. They might have to face a Buffy who is now their enemy. Also, the relationship between Spike and Buffy in these fanfics can be full of angst as Spike tries to either save her or deal with his new - found feelings for an evil Buffy. In some cases, the fanfics might explore the idea of a power struggle between them, with Buffy using her new - found evilness to overpower Spike in different ways.
One common theme could be a role - reversal in their relationship dynamic. Instead of Spike being the more 'evil' or morally ambiguous one, Buffy takes on that role. Another theme might be how Spike reacts to this new, evil Buffy. It could be about his struggle between his love for her and his sense of right and wrong.
Common themes in 'Buffy is evil Spike' fanfics include a distortion of the typical power balance between Buffy and Spike. Since Buffy is evil, she may have more power or be more unpredictable. There could also be a theme of betrayal. Buffy might betray the people she once cared about, including Spike. Additionally, the exploration of how the other characters in the Buffyverse perceive and deal with an evil Buffy is a recurring theme. It can range from trying to redeem her to simply trying to stop her from causing more harm.
In these fanfics, a likely theme is the exploration of family. Buffy and Spike are an unlikely pair, and having a captured baby in the mix forces them to deal with the idea of family in a new way. They might have to work together like a makeshift family to care for the baby. Also, the theme of sacrifice could be present. They may have to sacrifice their own wants or safety to ensure the baby's well - being.
There's a fanfic titled 'Evil Entwined' where Buffy and Spike's evilness is shown through their manipulation of other characters. They play mind games with the Scooby Gang, leading them into dangerous situations while laughing at their misfortunes. It's a really dark take on their relationship.
Another theme is the battle between good and evil within Spike. Given that he's a vampire, there's always that struggle, and Buffy's influence on his choices is a big part of many fanfics. For example, some fanfics might show how Spike's love for Buffy makes him question his vampiric nature and strive to be more 'human' in his actions.
Well, the theme of forbidden love is quite common. Given their different natures (Buffy being a slayer and Spike being a vampire), their relationship is often seen as something that shouldn't be. Another theme is the power dynamic. Buffy is strong and powerful as a slayer, and Spike has his own vampiric powers. How these powers interact and how they balance their relationship around it is a recurring theme. And, of course, there's the theme of sacrifice. They are both characters who are willing to sacrifice for the greater good or for each other in many fanfictions.
One common theme is redemption. Spike's journey towards redemption and how Buffy fits into that is often explored. Another is the idea of forbidden love, as their relationship has always had elements of being something not quite accepted. Also, the theme of sacrifice comes up a lot, like one of them sacrificing for the other's well - being or for the greater good.
Some people write such fanfics because they want to explore the darker sides of the characters. Buffy and Spike have complex personalities, and seeing them as evil allows for a different kind of character study.
One common theme is the love triangle. The complex relationships between Buffy, Spike, and Angel are often explored, with the characters torn between different kinds of love. For example, Buffy's love for both Spike's wildness and Angel's brooding nature.
One common theme is the exploration of their forbidden love. Buffy and Spike's relationship was always a bit taboo in the original series, and in fanfiction, this is often magnified. Another theme is the power struggle between them, which adds an extra layer of tension to their passionate encounters.
One common theme could be the relationship transformation. The idea of impregnation often changes the dynamic between Buffy and Spike in these fanfics.
One common theme is the struggle with their identities. Buffy is a slayer and Spike is a vampire, so having a baby challenges their usual roles. Another theme could be the reactions of their friends and enemies. Their friends might be shocked or supportive, while their enemies could see it as a weakness to exploit.