See, while I agree with you that Bruce would probably be a better man if he admitted that Cass could have done it and still be as devoted to the mission as he is, I don't think I'd love him if he did. One of the things I find most touching about Bruce is that when it comes to the people he loves, he can be as wrong as anyone else. And here is the sort of scene that makes it clear how much of an idealist Bruce is. To be fair to Bruce, IIRC, he never spoke to Cass of this at all, so it's not as if she had to be aware of how he thought of her. (I read two very interesting fics on the subject of Cass and Bruce and guilt.)
Otoh, Cass also had Oracle, who came to believe that Cass had done it and didn't think any less of her. Admittedly, there were other issues between Cass and Babs; if I had to draw a chart, I'd say that Bruce was the 'Ideal' end, with the mission and the scary bat god, and Babs was the 'Pragmatic' end, with things like her acceptance of Cass having killed someone and life as more complicated than just the mission (insisting that Cass get a life, and the issue of Cass not knowing how to read - which wasn't Babs' most glorious hour).
And as to what was better for Cass herself, well, I'll just be glad the two of them were there and she had Steph.