His cause was "wrong" because his side lost.

You bet, Sins, and Hitler & National Socialism were "wrong" because their side lost. Jesus, get a grip.

Actually, the gent in question was William Howard Russell, and his diary was written 1861-1862, & originally published in 1863, before the outcome of the war was known. This may color his descriptions & interpretations a bit. Subsequent (edited & revised?) editions published at the height of the "Lost Cause" nonsense.

However the soldiers comments at that time were correct, that [states rights]is what he was fighting for I am sure.

The old states rights dodge has been disproved so often its amazing that anyone still resorts to it. All anyone who wants to know which "States' Right", i.e. SLAVERY, the Confederacy was so exercised about has to do is read the South Carolina Ordinance of Secession (ditto those of other states).

Now, this particular Secesh may have THOUGHT he was fighting for some lofty general principle, but the common soldier is often mistaken about the real reason(s) for the conflict he's engaged in.