The essay for which the title is required obviously deals with the health risks surrounding fast food. It is difficult to choose a title without knowing the content but as the emphasis is apparently on the life-threatening effects of over-indulging in fast food, hopefully these observations will help.
The first title, "Eat Fast, Die Young," alludes to the well-known saying "Live Fast, Die Young." Although it has its first recorded origins in the nineteenth century, the saying emerged from Willard Motley's book, Knock on Any Door, and was popularized by the movies of the 1940s. Just as by living dangerously and taking risks a person can perhaps expect to live only a short life, fast food is not a wise choice if a person wishes to live a long and healthy life. This title, in my opinion, may be a little too glamorous to be taken seriously, although the fact that it does place responsibility on people for their own choices and actions makes it a possibility for a title.
Killing Me Softly with Fast Food conjures up images of Roberta Flack and her rendition of Killing Me Softly from the early 1970s. It sounds quite romantic instead of implying that fast food is subtle but deadly, almost unnoticed until it is too late. If the intention is to scare people into taking the fast food epidemic and the obesity that accompanies it seriously, this title may not be strong enough.
Fast Food: Faster Death? is short and to the point. An exclamation mark rather than a question mark would give it more emphasis and hopefully be shocking enough to convince readers that they need to keep reading. It is my preferred title and suggests that people should themselves be accountable for consuming fast food and recognizing the risks.
The next title may be a little confusing and be expecting too much of readers. It suggests that fast food bites the dust, not that the people who consume it meet their untimely demises. It does mean that those who consume fast food should be aware of its effects and that fast food then fails in its attempts to worsen people's eating habits, but (and depending on the emphasis of the piece it is intended to be the title of) it may not be strong enough if the intention is to persuade people to STOP eating so much fast food and to eat more healthy alternatives. There is also perhaps such a strong connection with Freddy Mercury and Queen's song Another One Bites the Dust that thoughts of fast food disappear from the radar.
In The Suspect—Fast Food, there is far more uncertainty as fast food is only a "suspect." If the essay then expounds on the reasons why fast food is a poor choice and gives reasons and evidence to support the view that fast food is the most likely cause of so many health issues such as obesity, heart disease and cholesterol, then it would be a good title. By the end of the essay, it would be hoped that fast food is no longer just a suspect but is sufficiently implicated to convince people to take responsibility for the effects of fast food.
Eric Schlosser's Fast Food Nation: The Dark Side of the All-American Meal, reminds readers of the many implications of fast food which far outweigh any personal considerations.
A title should be like a label on a can. It should tell me what is inside in the most direct and unmistakeable way possible. Asking someone to provide a title without having read the material makes the whole selection process arbitrary. My suggestion is that you read the work in question very thoroughly and determine a title that best "labels the can" for your readers, that is, let us know if it's dog food or chilli. Beyond that, your personal creativity creates the brand; however, keep in mind that your title is the first impression that your readers get of the work, so it needs to be something appropriate and a little provocative to pique interest. Hope this helps.