Revenge
It is obviously not a new concept for a characters arc and or goals.
Some champions were betrayed
Some experienced a great loss
Some based around inhibition power

It's not so perfectly cut and dry as this, as many of these characters are nuanced and have overlapping causes and rational for their current goals and state of mind. But many do share common story tropes such as 
I'm sorry to say this is another discussion on Varus and I know that it has been over done at this point but I wanted to analyze why the current changes that were made moved the champion in a positive direction that encourages a personal narrative rather than a shared identity with some of the other champions in the game.
As I saw it at the time Varus shared many story elements with fellow champions such as Vayne, Lucian, and Kassadin. It was as though his story was already divided three ways with these three champions hitting every beat of his own character with the death or absence of a loved one, vow of vengeance and taking on life altering path to succeed at their goals. This had been done before and at least two of these champions came out before Varus at the time.
Changing his tale to one of corruption as a means to prevent loss with a constant struggle for hope opened up his character and story for more growth and world building. You could argue that the Pit of Pallas could have just retained the Darkin and that would have allowed for more development as well, but at that point what would delineate Varus from Kayn aside from their reasons for seeking power. His story would share yet another identity and compared yet again to another champion.
If you believe it was controversial for Varus to have his sexual identity changed I would agree that there is a case to be made there, but I also see what riot was intending with this change. They were aiming for a story asking the question "can love conquer all," added with a loss of personal self and rediscovering hope in a hopeless situation. Romantic love in this case makes sense as it is impassioned and heavily relatable, the merging of two romantic individuals makes sense as it relates to marriage and other bonding ceremonies in society relating to 'eternal love.' If it was a family member or friend of some sort, the relationship would lack strength in conviction and emotional enthusiasm. Now you may ask, "Why didn't he just have a wife like before then, with this set up that would work just as well right?" And you would be right, but that brings forth even more questions about identity as gender would then become a discussion piece as well. How would you depict the wife in this new circumstance, how many voice actors would you need for this champion's new dialogue, how does this effect the gender of Varus and what does that entail for a game like league with a widely global player base? League of legends has had to censor their designs and champions before because of external conflict with other countries laws, and I believe their would be no difference here in this case.
Though it is always sad to see these characters that we have grown so fond of change sometimes entirely, there really isn't anything we can do but move on and accept the changes for what they are. I believe that they were done for the better and have both modernized the character and given them a solid direction for us to follow, and if you do not want to continue with this champion any further that is fair as well.
Best of luck to all that are currently conflicted or adverse to these changes, I hope that you can find your closure.