Amongst the assembled backstories of the various Avengers, Black Widow's is unabashedly the darkest. She's a child soldier, trained to be a weapon since birth - and who only really escapes this well into her adulthood, having had almost her entire life ruined - something that both convinces her to help others, and also leaves her with a ruthless streak that is integral to her character.
With this kind of backstory, it's hardly groundbreaking news that Black Widow has had some of the darkest moments of the cast of the MCU.
As such, it's pretty interesting trying to figure out exactly where Marvel will draw the line in the sand for the assassin-turned-superhero, as while it's impossible to not include some dark moments from her past, the sheer number of brutal and depressing scenes that have to be stepped around in order to use many of her major storylines is monumental.
Given Black Widow comics contain everything from child torture to surgery being done on a conscious person, there's a real wealth of storylines Marvel will have to dodge for the 2020 Black Widow film - unless they want it to end up with a surprise R-rating.