I would submit that suicide is "justifiable" when left to the discretion of the individual, ie., a "personal choice".
It is ultimately for others to objectively assess (after the fact) whether or not a suicide is "justified".
Personally, I regard suicide as a coward's "cop out", if a suicidal someone is merely having a bad day, or seeks to avoid unpleasant consequences from their own poor choices. Life is sometimes a contact sport. Get used to it.
Yet, it is easy to understand how some aspects of suicide can present a rational, reasoned choice...that is imminently "justifiable", and hence often "justified" (by others).
It was stated earlier within this thread the example of self-sacrifice (to knowingly place oneself in a situation where death is imminent or likely) to save/defend another (compatriot, loved one, complete stranger, or even a principle).
Arguably, the choice of self-sacrifice is not necessarily a "death wish" at all. Soldiers, firefighters, search/rescue teams, police, health professionals/care-givers, battlefield medics, etc., often put themselves directly into harm's (and prospective death's) course on a daily basis. It's probably fair to say that most (though perhaps not all) do not want to die, or wish to kill themselves...but virtually all place themselves in locations/situations that can most readily enhance the likelihood of premature death.
Is it "justified" to be war zone doctor; or to run into a burning building to save a child; or to enter a radiation-contaminated reactor core to prevent imminent melt-down, or to fight back against armed intruders into your home? Whether societally agreed upon or not whether such behavior/actions are "justified", they are most certainly "justifiable" to each individual that exercises a personal choice.
The main issue regarding matters of self-inflicted (or assisted) suicide is that of societally/civically imposed restrictions/laws that choose to assign what types or forms of suicide (ethically/morally/legally) are "justified"; regardless of any individual's own capacity (and right?) to make that determination and choice "justifiably" for themselves.
Many religions regard self-suicide/sacrifice as either a sin or affront to their god, somehow usurping or abridging that god's power and authority over matters of life and death. Personal ethics/morality/dogma are certainly understandable and (generally) acceptable when choices/behaviors are left to the individual to heed and keep for themselves. It's when religious adherents seek to project their personal morality (regardless of motivation/faith) upon others as a matter of civic policy/law that things like suicide become especially contentious. In a pluralistic, yet governmentally secular society, personal religious/ethical/moral choices should remain freely "justifiable" to each and every person, but never "justified" as universally applicable and criminally enforcible mandates of law over personal liberties.
Suicide ain't about sin.
It's about choice, and the freedom to exercise personal choice absent governmental intrusion. Our minds, and our lives, are the only possessions we can ever surely own...and should never defer nor abdicate personal choice and responsibility to the whims and wishes of government or any proscribed religious dogma.
You wanna die? That's cool with me...but just think of what you're gonna miss out on tomorrow. Life's too damn interesting to cut it short; but if you're just bored, or tired, or lonely...don't look for any encouragement or support from me. From my perspective...suicide is a choice you won't live to regret tomorrow.