You're going to get different answers based on the age of people for one thing. For another, based on their political views (Abe supporters, for example). Then there are the textbook publishers and the grassroots organizations to which people belong. Remember, too, that Chinese and Koreans make up a sizeable population here, so whether you ask a native Japanese or them, you are bound to get a different answer.
I'm guessing you outright asked people point blank, whether in a face to face situation or via a discussion room. Face to face, they will probably not feel comfortable talking about their real feelings unless they have a reason to trust you, and trust takes a very long time to acquire. Even in a discussion group, the anonymity can work against you, because usually only the very vocal will tell you their views (and I suspect it will be favoring imperialism) without fear that you will know who they are. You also have to be careful about anonymous discussion forums because you can't really say for certain who is answering, a Japanese or someone pretending.
If most Japanese people disown the war atrocities commited during the militarist Japan, why are there A-class war criminals still venerated at Yasukuni shrine?
Well, do most people actually disown the war crimes? That's the first assumption. Very hard to say. Moreover, it wasn't "most people" who built Yasukuni or conceived of it. That was a political move.
Why those criminals are still there is actually a multiple question, too.
1) Who actually considers them criminals? Some politicians will say yes/no and act accordingly. Some will hedge and say nothing, yet visit out of some sense of respect for their (other?) efforts during the war, despite being chastised by China and Korea every time there is a visit.
2) "Still there" begs the question: who decides that they should be moved, and to where? It's not something under active debate at all. It only comes up when the PM pays a visit.
3) "Venerated" needs definition, too, and this falls under
#1 responses, as well.
Complex issue.