Tolstoy from what I understand was directly involved in supporting the heretical Doukhobors and in organizing the financing of their emigration to Canada. The Doukhobors were extremists who rejected all of the New Testament except Matthew, and in Matthew reallly only cared about the Sermon on the Mount.
Once in Canada they objected vigorously to state schooling of their children and held nude protests. This led to public nudity being banned in British Columbia under statutory law (I'm surprised it wasn't a common law offence, but I guess in the past there were more occasions where there were legitimate uses for it...parading down high streets to make a political point au naturel not being one of them). There was a violent breakaway group from the mainstream Doukhobors community that relied on arson attacks to protest Canadian oppression into the 1960s apparently.
Note that up through the 1960s Canada was a fairly servere compared to rhe US. No prohibition, but corporal punishment was commonly inflicted on prisoners in federal and state penitentiaries, using a leather strap; at the turn of the century Delaware srill had a whipping post but I believe we discontinued such practices well ahead of the Canadians, as the definition of "cruel and unusual punishment" was tightened up throughout the 20th century. And in the 1930s the Canadians seized from their family quintuplets in order to exhibit them Ina sort of Zoo in Ontario somewhere I believe called Quinnland in order to cheer up the proletarian classes during the depression. And there was much unpleasantness for the Indians, or First Nations as they are now called. So it's very possible the Doukhobors had legitimate grievances upon settling in Canada. They should probably have picked the US at the time and I believe a Minority did move south, along with some Old Believers who initially tried Canada and are now in South Carolina, the so called "Wanderers."
But Tolstoys involvement with the Doukhobors, and his very enthusiastic support of them theologically and philosophically as well as practically, does seem to suggest a certain disdain for the Orthodox Church and especially her doctrinal approach in favor of a sort of Unitarian universalist ethos on Tolstoys part. I have even seen the word "Tolstoyism" used to describe his religious views.
So frankly I think St. John of Kronstadt was probably at least partially correct, in that Tolstoy was probably a heretic. We recognize John of Kronstadt as glorified for a reason, so if he identifies someone as a heretic we should heed that identification.
Now, was Tolstoy the worst heretic of his time? That's a harder question. He had stiff competition. Madame Blavatsky, Mary Baker Eddy, and Ellen G. White come to mind for starters. There was during his time a lot of Bohemian fascination with the occult and the sort of quietist universalist deism of Tolstoy and the Doukhobors seems a far cry from the excesses of the Theosophical Society. But I'm sure St. John was at least aware of them. He probably was not aware of the emerging American cults like Christian Science and the Jehovah's Witnesses and how many lives would be needlessly lost due to their strange views on medical treatment. But either way, Tolstoy doesn't seem very Orthodox to me at least, although there's no denying he was a nice enough chap.