To apply the scientific method here, I would just look at how many people choose to kill themselves, and how they choose to do it, and why.

Do they kill themselves with honor, in such a way as to truly end their suffering for noble, logical reasons?

Or is it just a coward's way out?

I think a lot of people do it the noble way, so it might truly be good and right to die if you are suffering an undue amount. It's natural to have dark feelings sometimes, so to deny them would be wrong, at least if there are truly legitimate reasons for wanting to end your pain, such as disease, life-horrors, or other things that can't be lived through, or survived.

I think the most logical criteria to justify suicide is: Will my situation ever get better? Or am I doomed, and cannot ever realistically escape my terrible, painful fate?

If asked honestly, many times a situation will get better, so it's most often not good to kill yourself. But sometimes it is. It's really up to each person.

No matter how a person dies, I think the right thing for the survivors to do is to honor that person and their life, and their choices. Respect for the dead is always important.