Mindfire
Istar
I'm guessing you haven't read it. They can be killed, and in fact one is in the first book.
What can they be killed by? If the answer is "other vampires" it doesn't count.
Yeah, because a perfected apex predator having a devastatingly fatal weakness that provides its specific prey a fighting chance makes so much more sense.
But vampires aren't evolved predators. They're more akin to demonic creatures or supernatural aberrations. How does something evolve if it's dead?
I'll make an exception for Underworld, because they've banked enough cool points to get a free pass, plus they at least make an effort at scientific explanation. If the vampires in Underworld evolved resistance to sunlight, it would still be kind of a cheat, but it could be an interesting plot point.
Twilight however does not have enough cool points to get a free pass. As far as evolution goes, sparkling makes even LESS sense than severe UV vulnerability does. How can you explain or justify diamond skin? You can't. It's just a cheap handwave to make the romance doable and to elicit swooning from the tween female audience. The only "evolution" Edward's skin represents is the ultimate evolution of teen idol-itis.
Also, vampires who can walk around in the sunlight without instantly bursting into flames has never ever been done before Twilight, ever. Well, except for Count Dracula, but it's not like he counts for anything, amiright?
Now let me explain why I give Dracula a pass (although in some versions light does kill him). In the rare instances when a vampire is immune to sunlight, it's supposed to signify something special, unique, and (most importantly) badass. Like Blade, the Daywalker. The ultimate vampire killer. The creatures of the night tremble at his name. Likewise Dracula is not just a vampire, but the greatest of all vampires, so if he is immune to sunlight it is because of his greater power and importance. Twilight commits a double sin. Not only is the vampire's immunity to sunlight made into a casual fact, but it is decidedly NOT badass.
Really, it's not the fact that they can survive sunlight most people are complaining about, but specifically that they sparkle. I just don't get it.
Explained above.
You and I clearly have different ideas of what counts as "boring." You know what I find boring? Being able to completely destroy an ancient, superhuman monster by walking up to the nearest window, throwing the curtains open and going: "It's a bit stuffy in here, don't you agree?"
It's not really that simple. Sunlight offers hope of surviving a vampire attack. It gives the humans a fighting chance. But it also creates a ticking clock. You have to make it to sunrise for that hope to mean anything. Compare that to twilight. If vampires attack, the humans are screwed. What can they do except sit around and wait for the good vampires or shirtless werewolves to save them. Yawn. That may work for teen girls who have a white knight/helpless damsel complex, but not for me.
Anyway, Twilight vamps can totes be killed, that's the whole reason they go to such lenghts to stay hidden. Aro, the head honcho evil vampire, specifically states that human technology had advanced to the point where they simply couldn't afford an outing.
(It doesn't help that they are absurdly flammable. You can seriously set these guys completely ablaze by throwing a zippo lighter at them.)
So yeah. Set their dwelling on fire, hit them with a rocket launcher, bomb their house, use ancient Native American magic to turn into a giant wolf and then bite their heads off, etc. It's doable.
If this is true, then the writer makes unforgivably poor use of it. AFAIK, Bella does nothing except get fought over. Plus, the fandom seems to think their sparkling darlings are totally invincible.