There are some ways I've seen it written.
- Pure narrator
- Pure dialog (telling it to someone else)
- In between
1) This one really depends if you're writing in first or third person perspective. Writing in first person, you would write it almost like a conversation, but only to the character themselves. So something like:
Oh, how did I get into this situation? That's right, it happened back then. Back when I decided humanity over barbarism. Back when I first met her, those eyes... so sweet and innocent. I couldn't have know back then. No. My heart wouldn't have let me know.
... and then go on to tell the story.
If it's in third person you could say the same thing, but with a less personal description and more of an observation description:
He wondered how he found himself in such a predicament. "That's right", he remembered, "it happened back then." Back when he decided humanity over barbarism. Back when he first met her, and fell in love with those eyes that were ever so sweet and innocent. He couldn't have know back then, or rather, his heart wouldn't have let him know.
2) A pure dialog would be somewhat similar to the first in first person, except you'd probably want someone to interject every now and then to actually make a conversation.
3) For this one, you would purposefully omit the character telling everything to their listener, while filling in the audience with narration. So like:
"What happened to her?" she asked.
"She died," he replied simply. And a horrible one too, he knew. And worst of all, it was by his hand.