As a character, Buffy the Vampire Slayer has one big, recurring flaw that she shares with other vampire hunters, and Spike can see right through it.
Throughout the course of the Buffy the Vampire Slayer series, the protagonist struggles with the weight of the world on his shoulders. Understandable, considering the former cheerleader was forced to go from humble 15-year-old to unquestioned protector of the world overnight. It's hard to deal with Buffy when she can't help but wear her blemishes on her sleeves.
However, Buffy is often in the habit of blaming all her character flaws and lapses on the stress of being a killer. It's easy to see how things go, but these complaints end up being all too easy scapegoats, even for someone like Buffy. For example, something as simple as Buffy choosing to have a casual relationship with Spike can't be reduced to just her making a choice for herself or wanting to have fun. Instead, as far as Buffy is concerned, it's an example of an anachronism that she wouldn't have done if she hadn't bowed to the pressure of Slayerhood. It ends up being a running gag that supporting characters know all too well, like in season 4's "Who Are You?" Faith (in Buffy's body) quipped Spike says that Buffy can "do anything I want. Instead, I choose to pout and whine and feel the burden of killing."
Spike is one of the few people who killed several Doom Slayers during their heyday. One, understands them and their inner woes better than most characters. In Sarah Gailey and Hannah Templer's Vampire Slayer #9, he explains his point to Xander, calling out Slayers and their tendency to attribute their flaws and misbehavior to their characters - and the way fate has dealt them - rather than Not acknowledging what's coming from their own personalities.
Slayers Don't Take Accountability
Granted, even Spike admits he says it like a pot says a kettle is black, but that's what makes him an expert here. Especially in how the franchise shows how he blames Angel himself for being a monster, Spike knows how easy it is to blame one's misfortunes on their circumstances rather than taking responsibility for one's actions. Readers can empathize with Bloody William's tragedies and how those tragedies brought him to where he is now, but at the same time, no one put a stake in his heart to compel him to kill many people He was killed or did the horrible things he did.
The same analogy can be made for Buffy, as previously mentioned, there is evidence in the original series that she would rather blame fate and the killer for most of her actions than take responsibility. Especially in the early seasons, because Buffy more or less gets out of that habit by the end of the show. Still, it doesn't make Spike's insults to Buffy and the other killers any less real. Buffy the Vampire Slayer, as a franchise, has repeatedly used fate as a scapegoat to present a recurring theme of its title character (and even other killers).