In Twilight vampires aren't evil at all. In Buffy some of them have souls, and others are a lot more annoying than really evil, and in the Underworld series the vampires are only morally grey at worst. They even specifically talk about not wanting to kill and eat humans, which separates them from the lycans.
To be more traditional about it. In folklore and myth vampires are deceased bodies which have arisen from the grave for any number of reasons. Common reasons include pacts with the devil, witchcraft and sorcery, or the sheer power of their hatred brought them back. So yes, traditionally vampires are all evil all the time, they have no souls and are merely a ghoul-like vestige of the person they once were.
When Bram Stoker wrote 'Dracula' this was the portrayal he chose, the Count was a vile blooduskcing fiend, evil before he even became a vampire, and one of the only characters we directly saw turned went and started murdering children afterwards. Again, soulless, bloodsucking fiends.
Mainly it's the fact that traditionally they are soulless, and without a soul one cannot have good and evil. In the olden olden days this was considered evil because only people of God had souls (hence why animals etc don't have souls either) and in the merely olden days this was because they were abominations of nature which shouldn't exist.
Vampires only really started becoming sympathetic after Hammer Horror got their hands on the idea, and started to inject first the class and sophistication, then the tortured soul routine. The traditional view of vampires is that they simply wear the clothes they were buried in, no matter how blood and dirt stained they got.
To be even more blunt about it, the modern interpretation of vampires only really took hold after Angel on Buffy was revealed to be a vampire with a soul. So Joss Whedon is directly responsible for Twilight, because without him vampires would still becloset lurking bogey-men.