I don't think you will ever really know. And it doesn't make any sense at all. It's a terrible thing to have happened to you and the family.
In a way, having experienced both parent's dying, my father of cancer my mother through suicide, I am not sure there is a reason you can find in any case where someone dies.
I think the control factor of suicide makes us want to go out and find the clues and solve it a bit like a mystery. But the fact my father got cancer was a mystery. A physical disease and death from it is not looked at in the same way but its really all on the same spectrum. I think its easier to track a death from a physical illness, but one from suicide, whether related to some sort of mental illness or not, is far harder. There is also something about our society that makes it all about the why.
Unless he said why he was doing it or left a note no one will ever really know. The inquest will find out quite a lot of things and ask questions and look for a reason in order to conclude a verdict and you might have some further answers from that process. I think some people find it a helpful process, others not but they do their best to try and find out the means and the method and the why of the death.
But the deep down 'facts' have sadly gone to the grave with your nephew. It was about him and he could have done it even if you knew him really really well.
