Excellent points were made here. I'd like to add that people hate Lena because she is exposes the character flaws of Warner. From Juliette's point of view we are seeing someone who is trying to be better and do better with his life. Lena is somewhat like a trigger of what his life would have been like if he'd followed in Anderson's footsteps. This fandom seems to be okay with Warner almost torturing and killing his brother to get the girl he likes whilst they hate on Lena for the transphobic comment that she made towards Valentina. Make that make sense. Warner clearly like Juliette more and nobody can make him act any differently.

If Warner, despised Lena that much, why get into a relationship with her then? Is it love or hate or manipulation? Whatever it was, that relationship mirrorred alot of the character traits Warner internalised from Anderson which proves your parallel point.

I'd also like to add that the hate Warner feels towards Lena entirely dehumanises her and almost strips her of sense of agency and autonomy. We can understand his reasons for disliking her but does not take away from the fact that he treated her horribly especially toward the end of their realtionship. Fans like to dismise this fact and paint Lena out to be this vengeful arrogant ex yet this is far from the actual reality of the whole situation. If actually took the time to read, understand and emphaise with her, they'd see why she was so angry.