Here is a great example of McClellan's issues.
Winfield Scott was in his mid-70s when the war broke out. He should have long-since retired. He had only five years left to live, was too fat to ride a horse, and was infamous for his vanity. He was an impediment to the Union war effort.
And yet Scott's Anaconda Plan, despite mockery from the press, was a good strategic outline of how to fight the war.
McClellan is simultaneously mostly correct, but also too dismissive - all while being a jerk about it.
Ironic that McClellan would criticize another general as "incompetent or a traitor" since that same criticism would be levied against Mac.
And yet those candid private remarks more accurately reflect your opinions and the "real you" than self-censored, carefully-considered public remarks you make on CWT or to your boss.
We don't know whether the Anaconda Plan would have worked, because it was never tried. There is a fundamental misunderstanding of the Anaconda Plan, because a key aspect of it was no military invasion of the southern states. This was unacceptable to Lincoln and to Chase, and hence they turned initially to McDowell and Franklin to work round Scott, and then to McClellan.
In the aftermath of Bull Run, Scott had a complete mental collapse. He ceased functioning as the army commander and retreated into fighting McClellan's attempts to build an army with the obvious objective of invading Scott's beloved Virginia. There was no effective General-in-Chief.
The secretary-of-war also took the Military Administration Act of 1836 and general tradition very seriously. He was not in the line of command and by regulations, in wartime, could not be.
There is an argument to be made, and I'd largely agree with it, that McClellan was acting well beyond his nominal rank in the period August-October 1861. However, he was doing this at the behest of President Lincoln. Lincoln bypassed Scott and Cameron and in ca. 1st August, asked McClellan to submit a strategic plan for the war. This he did on the 2nd, it being his "Strategy of Overwhelming," and that started the rift between Scott and McClellan.
McClellan expected the rebels to do the most sensible thing militarily, and concentrate a large army in front of Washington to attack it. This was well within the Confederacy's capabilities. That they didn't was because Jeff Davis was contending with "states rights" and couldn't get the states to release their forces to strike a blow against Washington. McClellan didn't, and couldn't, know this, and nor did many others.
The Scott-McClellan relationship deteriorated massively as Scott continued to frustrate McClellan's overly aggressive tendencies (as Scott saw it). McClellan became increasingly frustrated with Scott trying to block any action, and after three months resolved to have him removed, which he did. You might want to suggest this was insubordinate, but it was certainly for the good of the country.
We can have a certain sympathy for Scott, who had been ousted from role of General-in-Chief twice before, and spent the four years before the Civil War with no command assigned to his house in New York, i.e. exactly what would happen to McClellan in November '62. Scott was always highly strung and egotistical, and this certainly made him worse.
* Probably wrongly. The regulation predated the creation of the office of General-in-Chief, and clearly originally meant an army or department commander could form divisions and corps on their own authority. The corps aspect was removed completely in 1857, leaving no-one but the President legally able to form a Corps de Armee.