He would tell you that he's sorry he let you and everyone else around him down. That he didn't get to fix the bridges he burned, and mend the people he hurt.
That every time he came down from his high and was lying in bed unable to sleep, the guilt was like an ocean devouring him. That tonight would be the last time he would ever touch anything.
That he wanted to stop, knew he should stop, but when you wake up sweating & shaking all logic leaves your mind and the only thing you can think of is where to get more.
That it's not your (or anyone else's) fault, you did nothing wrong, and quite probably everything right. Some of us are just born with demons on our shoulder that won't stop whispering in our ear.
He would tell you of all the the plans he had come up with over the years, to make up the lost time with you and the rest of his loved ones. Doing things that YOU want to do, just to make you happy, because he had been selfish enough and you were owed at least that much.
That when things were their darkest, and he felt his lowest, one of the few saving graces and safe havens in his mind were the times he spent with you, before he fell into this pit of darkness.
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I am, genuinely, sorry to hear that. Not in a "my condolences way", but in a grief-ridden and deeply personal way.
I lost my father and several of my closest friends to fentanyl. I know your grief. I know the feeling of anger that also gives you guilt, that they should have robbed the people that loved them most of the opportunity to experience them, and robbed themselves of the bright future they were capable of having.
Nothing I, or anyone else says, can make it any better.
I shed tears while writing this. Nico mourns for you as much as you mourn for him.