I think the people who say suicide is a selfish act are seeing it through the eyes of the friends and family of the person who committed suicide, not seeing it through the eyes of the person him/herself.
To the persons who kill themselves, there is no other option. They're doing the world and everyone in it a favor by removing themselves from it. They have a skewed view of the world, they do NOT see the world as a "mentally healthy" person does, they are looking at the world through "depression goggles", and those goggles give a very warped view of the world to those who wear them.
Sadly, the only person who can take off those goggles are the wearer themselves.... no one else can do it for them. Recovery from the depths of such depression is possible, but only if the person takes the steps to do it. Sometimes the view through the goggles becomes too much and the person isn't able to take them off, and they make the very final decision to end their life. Depression kills people in this manner, the same as cancer and heart disease kill people.
Yes, the friends and family are the ones left behind to deal with the aftermath of the suicide. And that is horrible and traumatic. But know that the person who killed themselves didn't do it with the intention of causing you pain. They did it because they could no longer deal with their own pain, and that is an awful, awful place to be.