Lyric discussion by SheolCodeMonkey 

Cover art for Too Late lyrics by Black Sabbath

The song is clearly about someone making a deal with the devil, but I think it goes a bit deeper than that. These are Ronnie James Dio lyrics, and if there's one thing Ronnie loved doing with his lyrics, it's using magical and supernatural themes as a metaphor for something very real. I think this is about the more general theme of doing things that you regret and having to deal with the consequences, a metaphorical "deal with the devil", which is very on-brand for Ronnie.

Maybe you were being emotional at the time or didn't know any better, maybe you were scared and would have done anything to escape, maybe you were hurting and just wanted to make the pain go away, maybe you were angry and wanted to hurt the one responsible, maybe you were just lashing out and didn't mean it really. Maybe a lot of things. But it's too late.

All the regret in the world can't undo your past actions, no matter how much you wish you could go back and do things differently, no matter how much you didn't really mean it at the time, no matter how much you thought it was just a bit of fun, no matter how sorry you are.

We've all done things we regret, I know I have. I carry the guilt of the people I've done wrong by, and it eats me up inside every single day. That guilt is the Hell we condemn ourselves to with our actions, and now we have to live with it forever. The only thing we can do about it now is learn from it and try to be better in the future, be more considerate and thoughtful, make fewer mistakes, but even that won't erase the things we've done and the hurt and damage we've caused along the way. Maybe people suffered, maybe we threw away something we should have cherished and can never get it back. I'd give anything to be able to go back in time and stop myself from doing some of the things I've done.

But it's too late.

[Edit: Formatting]

My Interpretation