Undoubtedly Bellatrix is pretty much bad to her very core, and that's what we all love (or love to hate about her) but I think even Bellatrix can be a sympathetic character in some regards. And however minuscule I think there is some good in her but it comes out in a twisted way, that there are some redeemable qualities within her.

The main reason I can see Bella being a bit of a sympathetic character is because of her time in Azkaban. A drab, dreary, miserable place filled to the brim with sinister creatures that suck your soul out and rip away your memories whilst simultaneously making an already chilly prison cell grow frigid. Of course she bought this one upon herself but it's still a grim fate no less. A place that can drive the sane completely batshit.

And Bella was already unraveling. Which brings me to the next point; Bellatrix has always been unstable and not one person thought to intervene in any way. And if they did, the notion of going up against such a powerful family to get her some help was probably off putting. I find Bella to be sympathetic in that she was very much predisposed for evil. I mean it's one thing if the characters had done their best to try to put her in an institution where she could get help and it just didn't work out and another for them to just throw her right into Azkaban. Because there is very clearly something damaged in her. If she wasn't messed up before because of Azkaban she's pretty much beyond help now, I mean its pretty hard to repair a person's mental state after the person had been subjected to forgetting all good memories and having the bad ones play over and over and over. Not to mention the sheer terror of the look of the dementors alone. Furthermore as you mentioned Bella also already had a shitty upbringing I can't imagine that Azkaban would help her to feel those happy fluffy forgiveness feels. And then there's the fact that these people have magic; how do they not have advanced mental health care. I mean they have luck and love potions. What about something for people like Bellatrix and Sirius and even Tom Riddle? Or idk, Alice and Frank Longbottom...
I'd really like to hear the workings behind all of these things. Like is there some kind of road block that prevents magical medications?
Long story short I find that Bellatrix is sympathetic due in part to her mental aliment and the Wizarding World's poor handling of the mentally ill.

I mentioned her upbringing above. I do believe that this is another way in which Bellatrix is sympathetic. Based on the way Sirius spoke of his family I would imagine that it wasn't a healthy (at best) and unsafe (at worst) environment. It is a personal headcanon of mine--based upon Sirius' commentary--that Bella and her sisters were subjected to some type of abuse or another be it physical, mental, or both. If Rowling ever dives into it and this proves to be the case then there is definitely sympathy earned there.

Finally Bella's unrequited love has racked up some pity points. She spent 15 years in Azkaban and would (and did) die for him. But he can never truly return her affections due in part to how fractured his soul is. Sure she was one of his favorites but she was his favorite in the same way a chess master has a favorite chess piece. She was a pawn. Yes they had Delphi together but I feel as though their sex wasn't the product of love (not on Voldemort's end anyhow). For him it was more or less; she did some good things for my cause so I guess I'll reward her. For him it was nothing beyond sex, no feelings involved.
Not only this but I can't imagine that Voldemort treated her well after failures like not getting the prophecy. On various occasions she was subjected to verbal abuse like "I didn't come here to hear your sniveling" and the bit in Dark Lord Ascending where he calls out she and the Malfoys. In the movies we see her getting physically abused by him as well--this is scene when he shoves her over after she offers him a helping hand. I can imagine that the things that go on behind the scenes are less than pleasant and it completely wouldn't surprise me to hear that he sent a crucio or two her way at one point or another. And she's not gonna fight back either because she loves him that much.


So what kind of redeemable qualities do I see in Bella? I see loyalty, I see confidence, and I see strength.

Say what you will about Bellatrix but she's loyal to the bone. She has the loyalty of a Hufflepuff. She's a ride or die kind of woman. I've already discussed her 15 years in Azkaban; ya gotta be loyal to stand before a court and declare, "The Dark Lord will rise again, Crouch! Throw us into Azkaban; we will wait! He will rise again and will come for us, he will reward us beyond any of his other supporters! We alone were faithful! We alone tried to find him!" She was one of the few that didn't try to worm her way out with the imperio excuse. She may be loyal to the wrong causes but the fact that she's loyal at all is redeeming to me. After all she could be like Lucius; loyal to Voldemort until the trials roll around.
Bellatrix is also fiercely loyal to her family. I feel like she has a special loyalty to and care for Cissy. I mean she did accompany her to Snape's instead of just letting her go off to do something she thought was dangerous alone.

I suppose Bellatrix's confidence borders more on arrogance. But I still think that her demeanor is very respectable. Voldemort aside she doesn't let people walk all over her. And hand in hand with that she's a very fierce woman. The kind of character who can handle herself without needing to rely on anyone else. I say needing because I think she wants to rely on Voldemort. But without him around she can definitely hold her own.

I suppose I hold a rather unpopular opinion in that I think all of the above. But I figured I'd share no less.