arabella, I understand where you are coming from. The situation Buffy was in wasn't pleasant. She had to choose between her death or the death of her sister. It was made clear to the audience even before the fight went down that Dawn would not die and if anyone tried to kill her (even a scooby) she would kill them.
She knew deep down inside that if it came down to it she would die for her sister. I think her death being a "suicide" or a "sacrifice" is under the definition and understanding of each individual viewer. I believe it was sacrifice, You believe it was suicide. So, let's just agree to disagree. 😛