Why doesn't Edward want Bella to become a vampire at the end of the Twilight movie? Traditionally, becoming a vampire would be pretty gruesome. However, Stephenie Meyers vampires are far from traditional. In the Twilight world being a vampire would have it's advantages.
First, Twilight vampires are not harmed by the sun. This means they are able to walk around during daylight. It's how the Cullens attend high school. The do live in the rainy Pacific Northwest because they glow in direct sunlight. The glowing would be difficult to explain, so they only go out on overcast days. You could still be in the sunlight as long as humans do not observe you. Since the Cullens play a private baseball game, you can imagine a private sunlight excursion.
Second, the ability to withstand daylight means Twilight vampires are not asleep during the day. In fact, sleep is not necessary at all. There is no coffin or dirt from the original grave that Edward must rest in.
Third, the Twilight vampires can survive on the blood of animals. This important fact means the Cullens do not murder people. Edward describes himself as a vegetarian. Edward does struggle with his desire to kill Bella and drink her blood. However, he doesn't have that problem with anyone else. Meaning the likelihood of him loosing control around other humans is remote.
Fourth, the Twilight vampires are immortal. Edward does not age, but remains a youthful man, fit and in his prime. Edwards diet is also unlikely to make heavyset, so he will remain youthful.
Finally, the Twilight vampires have strength, speed and various other super human abilities. Rosalie has an ESP like clairvoyance, and other Cullens are said to have gifts.
In the end becoming a Twilight vampire seems to have more benefits than drawbacks. The only two drawbacks being that you must live drinking the blood of animals and that you must socially avoid direct sunlight. In return you get immortality, you no longer need to sleep, you gain super strength and speed, and you may have some gift like Rosalie.