It is in some religions. I think it depends on the circumstances. If someone is suffering from a terminal illness, that's not as bad as some young person who kills themself just because their girlfriend dumped them.
I don´t think it´s a sin (the term is s.th. I don´t believe in).
Everyone is free to decide when and how they want to die. We didn´t ask to come to this world, at least we should have the freedom to decide when to leave it.