First off, Kalashnikov never made any money from the rifle. Russia had no patent system, and gave the design for free to all alllies.
"Inventor Mikhail Kalashnikov only earned Soviet wages for creating the AK-47 — but he hoped to cash in on globally recognized brands of Kalashnikov vodka — for which he licensed his name to two vodka makers. In the last years of his life, Kalashnikov made most of his money doing celebrity appearances at arms shows."
I think it was when he got older, 60+, he started making these comments. I am not an expert in Kalashnikov in any way, I just find it interesting that the man who designed probably the best rifle in the world, wished he had made a lawnmover instead.
I think that, with age comes wisdom, and insight. With wisdom and insight comes regret for the things you wish you had done differently, or said differently. And knowing that you have been the main cause of death for 100.000s people might weigh heavy on your conscience when you grow one.