Ultimately, what we're talking about is creating an infectious thought that causes people to value death over life. So there are really just two ways to do this:
- Increase the value of death, suicide, or depression-inducing behaviours.
- Decrease the value of life.
Both of these are real and serious dangers, and things we have observed in the real world already, as others have already observed. The modern concept of jihad is not a natural aspect of most Islamic theologies in its commonly understood form, but is a possible extension of some of them. In this specific case it is essentially accomplished by affirming the idea that only a glorious death in service to God can ensure eternal life. This isn't necessarily a far cry from some Norse, Spartan or Japanese concepts of a 'glorious' or 'honourable' death.
On the flip side there are subcultures like those people refer to as emo or goth, which glorify depressive behaviours over positive ones. If you're able to affirm the worth of such behaviours to the point where they are more overwhelming than those which encourage people, then you gradually create an oppressive culture which will increase the risk of suicide.
Thinking back to the memetic property, this would probably take the form of a story, a poem, a song, a video or popular saying which reinforces ideas of death or depressive-type behaviour being a good thing.
Make life worth less. It's a tragedy and a travesty that as Westerners we really don't have to think too hard about this one - what are the main things which people draw personal value from? Therefore to undermine people's self-worth and increase the relative perceived value of death, the following would be fairly effective:
- Undermine the integrity of family and marriage, either specifically or generally. If people can't trust their own families to stay together and can't trust another person enough to commit themselves to them, then their perceived self-worth falls.
- Undermine their sense of identity. Identity is a person's sense of ongoing self-sameness. If somebody does not have a strong sense of being the same person from one day to another, and has people affirming that they are significantly different from who they think they are, then this increases depressive thoughts.
- Undermine the basic value of life. Normalise death. Encourage death as a good and positive and normal thing for old people, adults and children alike, especially for those with less perceived worth. Too old? Too foreign? Too disabled? Too young? Too helpless? There are a thousand categories you can choose. But undermining the basic sanctity of human life at any age inevitably ties life to worthiness. And if all people are not basically worthy to live, then everybody is left asking themselves that same depressing question: Why am I alive?
- Undermine their sense of community: people are basically social animals. If you drive people into singleness and loneliness, then they will struggle to sustain their own happiness, and miss out on all kinds of affirmation from other people. If people don't form strong bonds with other people or don't value forming strong bonds with others, then they will probably become depressive unless there is another particularly strong sense of meaning in their worldview.
- Undermine any sense of objective morality, i.e. divinity. Objective morality depends on an over-arching schema which gives things meaning from the outside. In most cultures God is the one who gives all things meaning, and so people draw meaning and self-worth from him. Undermining this concept and removing objective morality and objective worth can rapidly deteriorate somebody who has learned to rely on this for meaning.
- Undermine their sense of reality, and invest their identity or self-worth somewhere else. This happens already! If a person invests too much of themselves in an alter-ego to the degree that they neglect their physical self, then they can die of self-neglect.
- Create an unrealistic standard of 'worthiness' in life. If you can create a wonderful and noble and glorious standard of life to which everybody should always attain, and reinforce that this is the most important thing everybody should do, then that becomes depressive. It's the most common reason people quit Facebook - because of this perception that everybody is happier than me. If the basic standard required to make life worthwhile is higher than most people's actual or even possible standard of life, that creates a depressive setting for the majority.
Disclaimer: I personally disagree strongly and passionately with all of the above ideas, and only present them because they are true answers to the stated question of increasing depression and suicide among people.
In a worldbuilding setting, a central memetic agent could hit all these different marks, and to do so in much more overt ways than western cultures do today, whilst still being subtle enough that it's not obvious to the consumers that this is their objective.
It would be really easy to create a story which is simple, believable and larger-than-life, because it really isn't too much different from the world many of us already live in! Isn't that simultaneously sad and encouraging for you as you consider creating your story?