Both are pretty messed up, but it seems the people who said the stuff about Pearl Harbor seem to fully realize that they are being insensitive, and it looks like to me (based on the screencaps below) that they are trying to come off as assholish and shocking.
The Hetalia fans mentioned in this secret, though, just seem to be too naive to realize how offensive they are being. They look to me as if they are stuck in their own little fantasy world and aren't even thinking about how insensitive they are being. They just seem to me like they are too ignorant to fully realize what they are doing.