A total mental breakdown does not have to make any sense.
No, but all murders of this type (slaughter the spouse and children, with or without slaughtering oneself too) obviously involve a total mental breakdown. Yet all the other notes I’ve seen reported offered some sort of “reason”, and certainly weren’t prepared in duplicate. Of course, many don’t leave notes at all, because in their mentally broken down state they either can’t articulate anything or just don’t care what anyone thinks. I wouldn’t expect anyone who did something like this to leave a “rational” sounding note, but if there’s a note at all, you’d think even an irrational mind that had enough energy to commit such a horrific crime would be able to come up with something beyond the “I did it; I suck” theme of this one.