There are a couple of things going on here, I think.
First, stories of missed chances tear me up. That quote about he saddest words in the English language being "it might have been"? Yeah.
The better the opportunity -the greatest the tragedy that could have been averted- the more I get angry or sad. This one, putting together Cass and J'onn (two of my fav characters) and then having them pass by each other like the proverbial ships in the night... I realize that properly speaking, I've written far more disturbing things. But this saddens me in a way my usual stories about broken people and destroyed worlds don't.
I could blame DC for it, too (heh, couldn't we always?). Cass has -perhaps- gone off the deep end, brought there by what I can only call serial abuse and negligence from a long and scary string of parental mentoring figures. Cain, Bruce, Babs, Tim - she has gone up in the sanity scale (one could argue), but apparently not high enough to find somebody that will
1. Teach her to read/speak/socialize.
2. Sit with her and *talk* about random stuff.
3. Generally speaking, you know, be human with her.
She has been alone all of her life. Isolated, in a pattern that is not deliberate-compulsive like Tim's, or post-traumatic-compulsive as Bruce's or Babs'. She is, for lack of a better image, the more innocent of them. The one that seeked contact. Which, in that group, is more or less like seeking liquid water in Mercury.
J'onn might have been good for her. If nothing else, J'onn might have led her to a place where she could find that and grow a little less scary and a whole lot more sane.
Instead, they never met unless they were playing their caped roles, or they did and... and they didn't understand each other. Happens, even with telepaths and body readers.
Still, it's sad. I guess I'm just sad for Cass.
(Also: I'm kind of generally sad tonight, so in hindsight it's not strange that I'd write or meta like this. By tomorrow evening I'll be back to writing Five People Cassandra Cain Didn't Kill and cackling while doing it, most likely.)
ETA: There is also the fact that Cass wouldn't, most likely, understand or agree with what I'm talking about. From her POV, being Batgirl, working for the Bat, fulfilled her in a very real sense. She is what she is, does what she does, and feels justifiably proud of that. On the other hand, there is also everything she could have been and isn't. Drawing a perhaps forced parallel, Cain was her Crime Alley, but I'll be damned if Babs and Bruce didn't play enabler!Alfred with her. Just because Cass didn't know or see a different way, it doesn't mean they shouldn't have at least offered her other possibilities. That's what adults are for, and while Cass met the Bats relatively "old" as these things go, they were still the first nominally white hat environment she had lived in.
On the other hand, look *who* I'm asking for mature parental attitudes. I probably need to go to bed now, before I start bitching at Lex for obsessing with Superman instead of just becoming a bigger hero than him.