There are. Japanese soldiers often wrote diaries, and this was no exception. There's a site where you can see some snippets of these, if you want a more general collection without tracking down individual diary publications.

That site is managed by Dr. Masato Kajimoto at UHK.

Those accounts are transcribed from interviews conducted by Ono Kenji who collected a number of accounts in the 1980s.

The short answer is that at the time, the Japanese soldiers were (in general) not too bothered by what they were doing, though a number recognised after the fact that it was pretty terrible.

As an example:

Well, in retrospect we were crazy. When I was taking them [to the execution site], I knew I was going to kill them. But I didn’t feel any guilt. Of course I can’t imagine doing it now.

Many were bothered by their actions later in life, but at the time, the racism and indoctrination were pretty effective.

On a sort of historiography note, there's a Chinese film that came out a few years ago called 南京南京 in which one Japanese soldier is shown in a more empathetic light. In China when this film came out people were outraged at the director for having done that. This Japanese character is obviously not an actual person, but you do get the sense from soldier diaries that such expressions would have been pretty uncommon at the time.