There are other tests.
But Lee was in favour of the Union.
Unfortunately for him, the civil war divided it, and he chose the side which included the state from which he originated.
Once the war was lost, and Virginia's interests were overcome, he pursued his second priority which was to reconcile the two sides. So he passed your writer's test in the most trying circumstances.
But again, the hard questions involve ideals in opposition. You can't isolate either the union or slavery. You look at the whole bundle of ideals involved in the argument and figure which things matter to you more.
If Lee had been borne in Connecticut, he would have been a Union officer according to his list of priorities. Neither he nor Lincoln prioritised emancipation to begin with, so there's no major difference between the two of them on this matter. For both of them, the union had priority.
For me, the confederacy's strongest argument was that states should be entitled to secede. But neither Lee nor Lincoln offered that argument when their decisions were made.
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