I am so sorry for your loss. Reading others comment on their own experiences losing loved ones to this disease, is chilling and makes my heart hurt. This disease is particularly cruel because those afflicted have it in their physical control to save themselves before it gets too bad, yet so many lose the battle regardless of this fortune. And it doesn't matter what we do which makes it all the harder. We can take care of them and love them and provide and do all the work to get them every possible tool out there, every tool proven to be successful, but in the end we cannot save someone and in some cases, they are not able to save themselves.
In these times, we look to God and are forced to make peace that some things are painfully unexplainable. Why does someone do this - why couldn't they just fight harder? Why did they need this poison so much when everything good was at risk of being lost and in fact, was? We as survivors and friends and family of the addict, need to surrender ourselves - just as the addict does and accept that we simply cannot make sense of it all. And that is ok.
As you said, he is no longer in pain anymore and perhaps, in a new life embodied as something else free of that pain he could not surrender to in this world. Perhaps there is another world where he is no longer suffering and instead flying freely, demons gone.
Much love and big arms of support to you and to those in your shoes past and present. It's okay to grieve...you've fought hard.