I have a character with a grim past (forced to watch torture). I need him to have that grim past in order to explain his behaviour in the story I am writing now: characters who did not know his past are taken by surprise by how nasty he is willing to be if it will help bring down the regime that did the torture.
The trouble is that I'm beginning to feel the whole current story, which isn't nearly so grim or black-and-white as the backstory, looks trivial in comparison. I fear that anyone reading it is going to diagnose a case of what TV Tropes calls "They Wasted A Perfectly Good Plot"
I have zero desire to ever write the grim stuff as a separate story. It's too depressing and also too monotonous.
Nor do I want to simply delete his backstory, or scale it back drastically. It would mess up my whole picture of what he's like. The way that the backstory is overwhelming the main story isn't really a matter of how many words are devoted to each; it's more a matter of how much more emotionally charged the earlier events are in comparison to the events of the main plot.
Any advice on how I can square the circle?