I am not an expert nor a Mormon, but was raised in a strong Mormon household (I'm what some refer to as an "ex-mo," meaning "ex-mormon").
My brother committed suicide when I was little and he was a teenager, so this was a discussion that came up a lot in my family. It is my understanding that murder is unforgivable/"unrepentable" because it is impossible to make the situation "whole" again, because the person is dead. Like, there's no way to make restitution.
I had a difficult time as a five-year-old understanding why any God would punish my brother for what I already at that age recognized as some sort of mental health issue, so my mom told me that God (Heavenly Father is how Mormons refer to God) knows all and would know what was in my brother's heart, etc. etc. I don't know if that's true, but I suspect it was just to make me feel better.
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I snipped your post to save space but wanted to say that I am sorry that, as a 5 year old, you had to go through losing your brother to suicide. Just so sorry.
The honest answer, as I understand it, is what you mom told you. We believe strongly in the sanctity of life. We do not believe it is ok to take your own life or that of another.
Having said that, I, too, have a dear friend who lost her son to suicide. After his death, they found out his medication was wrong and likely caused him to take actions he would not have taken had he not been on that medication.
Also, as an attorney who has dealt with children experiencing severe mental illness, I can honestly say that some of these children have significant burdens that are not their choice to carry.
Since every situation is different, the official position is that we are not able to judge. There is no regulation or directive that says those who commit suicide cannot have eternal life and all the blessings. Our Heavenly Father dearly loves each of his children. Those who carry enormous burdens are loved and adored by him. Thus, while the doctrine is that we are very much discouraged from suicide, we have love and compassion for those families impacted by suicide knowing that God will judge each situation with fairness and love.
Again, I am so sorry about your brother. I know that my friend's family carries great pain from her son's loss. He was very special, as I am sure your brother was, as well.