Sasuke is in a terrible place right now, and, I think if Saya would let him, then I think he'd want to help her with her fight against Diva. His plans already got shot down and murdered. He doesn't want hers to share the same fate.
Since he already knows how important it is, he likely would not aim for fatal blows against Diva. That's Saya's job. It's important that she's the one who finishes it.