To get into why Black Widow, IMHO, is horribly misused lets look at her origin story. Whom started out as a spy whom worked for the then Soviet Union and was brought up in the Red Room to become one of the most dangerous spies/assassins that ever existed.
Hell her history goes as far as the World Wars. Where the Nazis killed her family then she was raised by russian scientist Ivan Petrovitch, and became responsible for turning Natasha into the Black Widow we see today. There is also in account the history that she had with Captain America, Logan, and Winter Soldier during that time.
And speaking of the Red Room lets take a look at her training there shall we?
Much more interesting than that crappy ballerina flashback in Age of Ultron. To put into perspective imagine the movie Hanna.
Just saying, isn't just me, or have the MCU really neutered Natasha's origin? Hell they went as far as to waste what could have been a good backstory just for the sake of turning it into a romance subplot with her and Bruce. While in return turning Natasha into a designated love interest.
I could name some noteworthy scenes from MCU's Black Widow.
Then there is what the Russos have done with her. Yet when people think of MCU's Black Widow they think of her as the person that got shipped around alot to the male Avengers, sans Thor, or complain how weak Age of Ultron made her look. Which she was. Or how the character is nothing more than an eye candy device. And don't get me wrong I love me some eye candy but Natasha is more complex than that. In a way she's the closest to the female equivalent of James Bond but with her own share of inner demons still haunting her. And whenever those are explored in the MCU its usually just a way to spurn some romantic plot device. Rather than seeing Natasha trying to overcome her personal demons.
I mean lets look at say characters like Jessica Jones. We actually get to see her struggling against her own inner demons and while there is a subplot involving her and Luke Cage she wasn't regulated to just a love interest. Jessica Jones is a strong character both literally and figuratively while also showcasing a vulnerable side that she succeeds in overcoming on her own than depending on a man to save her.
And the saddest part concerning MCU's Natasha is that she haven't gotten her own solo movie. While Captain Marvel could be getting one before her.
With Civil War coming up heres hoping a better character comes out of MCU's Black Widow because she has potential to be better than what we are seeing so far.