That doesn't work for me as comparable to what happens to Buffy and Willow because those things all happen to Anya and are the result of her choices. Xander is a bystander, and any hurt he feels results from his own sense of entitlement (e.g., Anya shouldn't sleep with another guy, etc.).

But Tara being murdered is something that happens to Tara. And while it's not a direct result of Tara's choices, neither is it a direct result of Willow's. It's a result of Warren's choice.