Lincoln's "compassion" is irrelevant, inasmuch as he was assassinated five days after Appomattox. The new U.S. president, Andrew Johnson, was a personal enemy of Davis since their days together in the Senate. Nor did Johnson share any of Lincoln's "compassion" toward the South.
The Johnson administration very much wanted to prosecute Davis for treason. It hired respected jurist Francis Lieber of Massachusetts to prepare the case against him (and, for good measure, to see if he could link Davis to the Lincoln murder). Lieber spent months poring over Confederate documents and could find absolutely zero evidence connecting Davis to Lincoln's assassination. So then the administration fell back on the charge of treason, and was advised by Lieber, "Davis will not be found guilty and we shall stand there completely beaten." (Foote, The Civil War, Vol. 3). Davis was released from Fortress Monroe after one year in solitary confinement.
The unconstitutionality of the doctrine of secession was not, in the mid-19th century, a cut-and-dried notion. In fact, when Mississippi seceded from the Union on Jan. 9, 1861, then-U.S. Senator Jefferson Davis did not immediately return to his home state, as did his other Southern colleagues. He stayed in D.C. for almost two weeks, hoping to be arrested for treason, thereby putting the constitutionality of secession in front of the courts...and possibly averting the war. The arrest never came. While Davis was incarcerated at Ft. Monroe, he spent much of his time preparing his legal defense, which of course would be a constitutional defense of secession. He never got the opportunity to use it.
Except for a backhanded slap at secession by a Reconstruction-era SCOTUS (1869) in an obscure bond case (Texas v White), the constitutionality of secession has never been specifically litigated. Until such time as it has, any reference to Lee, Davis, or any other Confederate as "traitors" is merely personal opinion without judicial concurrence. Surely you can understand how some people might find that offensive.