Sakura with the shadow abilities but without the corruption of Angra Mainyu is a pretty cool character, though. I certainly like that idea. And Sakura in Hollow Ataraxia can fight, and Sakura following HF could certainly learn (and she'd be damn powerful, too, because she has the connection to Akasha still).
Yes, she killed Shinji. Although, he was trying to rape her at the time, and it was kind of an accident (she sort of 'wished' he'd die, and then he did).
But Dark Sakura isn't really nice. Some of the things she does are definitely not, especially in the last bad end (she swallows Rin, and then inflicts her own past on her, to show her what it was like). Dark Sakura is definitely evil, she just has the voice of the normal Sakura inside her acting as a brake, to control her really bad impulses. Dark Sakura is actually weaker and more conflicted than normal Sakura. It's what Sakura turns into when she can't resist the impulses from Angra Mainyu any more. She's got more power, yes, but she's not a stronger person.
i like dark sakura more then sakura. she can fight and that is cool but she is nice and its true that she stops herself. but did she kill her brother ??
If you mean "are Sakura's shadow powers awesome" then, yeah, I guess they are. But, if you mean "do I like the psychotic distortion of the sweetest girl in the game" then, no, not really. Dark Sakura is damn good at fighting, but she's not a particularly nice person, because of the corruption from Angra Mainyu (as the last bad end shows). Only Sakura's true nature stops her going totally over the edge, with the exception of that last ending, where Rin pushes her over it with her (apparent) complete lack of compassion for Sakura.
I like the idea of a Sakura who can fight like that, but has her usual kindness, though (but not necessarily her submissiveness, which comes from being abused by Zouken for so long). That would be awesome. Post-HF Sakura might well be like that, actually, at least to some extent.