They know they are killing themselves. Some people are certain that no matter what day it is, they have six months to live from that day. They cannot plan a future or take anything or anyone really seriously since they don't believe they will be alive long enough to follow through on anything. When you discover that someone you know is bulimic or anorexic, you may question your criteria for evaluating the world and the people in it. Most of the time you can't easily identify someone who has an eating disorder.
Some people are skeletal.
Some are normally weighted. Some are a little overweight. Some are fat. Some of these people having eating disorders. Some look they way they do for other reasons.