I think that is somewhat unfair. I don't think any of them, including Lincoln, thought that stopping independence for the South would cost them 750,000 (the latest updated number) lives.
All of their behavior indicates they thought this would end quickly and with little bloodshed. They were shocked that the Southern defenses were so strong, and that the Southern will would be so determined.
What they thought would be a cheap and short war turned into a massive disaster.
I will point out that often when we have had "super genius" leaders, they tend to make the sort of blunders that a less "brilliant" man would not have made.
I am very cynical of politicians and elitists, and I tend to think they are malicious, before I think they are ignorant and stupid. They may be that too, but most of them have evil intentions.
Evil is a pretty simple way to describe their motivation. It is not entirely accurate, but it is close enough.
That's probably true too. I don't think they anticipated the utter disaster it turned out to be.
I think World War 1 was similar, in that many of those people thought that would be over quickly, and it also turned out to be a massive disaster.