In the first instance, a person has insulted an already depressed person by calling them selfish. They have armed them with an additional reason to feel worthless and pushed them one step closer to following through with their plans to end the misery.
In the second instance, the person has invalidated the feelings and situation of the suicidal person, made the depressed person responsible for their well-being, added to their sense of worthlessness, added additional stress to their situation, and focused the struggles of the suicidal person on themselves.
There is a selfish person in these scenarios, but its not the one contemplating suicide.
Many Christians believe that suicides go to Hell. I cannot find any concrete evidence in the Bible to support this belief. Perhaps the person in question believes this teaching. They are willing to risk an eternity in Hell in order to escape the hell that is their life. Calling them selfish and invalidating their feelings is the most selfish thing someone can say to a suicidal person. They might as well pull the trigger themselves.
A person who commits suicide is only selfish to one person: Jesus. A person who commits suicide is willing to risk the very hell that Jesus died to save him from. A person has no right to call a suicidal individual selfish unless they have personally died for their sins.