Unquestionably, Lincoln's decision to issue the Emancipation Proclamation was a tactical decision rather than a humanitarian act. Although many historians have embellished Lincoln's motives, Lincoln was not all that concerned with the humanitarian element .
It is not coincidental that the Emancipation Proclamation was issued shortly after Lee's invasion of the North that had been stopped at the Battle of Antietam. Lee's intention was to prove the strength of the Confederacy and hopefully gain recognition from England and France. Lincoln knew of European aversion to slavery, and by issuing the proclamation, he foreclosed the possibility of European intervention in the war.
It is also not coincidental that the Proclamation only proclaimed slavery in those states in rebellion. Slaves in the "border states" such as Maryland and Delaware were not freed. More telling is the fact that certain counties in Virginia and certain parishes in Louisiana were excepted in the language of the Proclamation; this was because those areas were under Union control.
Interestingly, there is no language in the proclamation that deplores the evils of slavery. One would expect such language if Lincoln's motives were altruistic. So before we place Lincoln on any higher pedestal, we must remember that he was a wartime commander determined to save the Union. The Emancipation Proclamation was an attempt to further cripple the Southern effort and thereby bring the war to a speedier conclusion. An end to slavery forever was not in Lincoln's mind.
Lincoln deplored slavery and believed if its spread into the west was prohibited, in time, slavery would eventually die out in the south. It was for this belief that John C. Calhoun swore if Lincoln was elected South Carolina would secede from the Union. Other southern states followed and within a few months of the election the nation was ripped in half. Lincoln believed in the preservation of the United States and from his own words 'would use every means necessary' to accomplish that task. Although your question asks whether Lincoln's decision to abolish slavery was a war tactic or humanitarian act it negates an extremely important point; Lincoln never abolished slavery. He had no Constitutional authority to do so, and he knew that. (only a Constitutional Amendment could legally, abolish slavery in the U.S.)
So Lincoln carried his Emancipation Proclamation in his pocket for months, waiting for a Union victory before he made it public. This action suggests Lincoln's motives were rooted in war tactics. However, make no mistake Lincoln understood full well that his Proclamation would carry moral humanitarian weight. With no legality, no federal authority, the only power Lincoln really had regarding slavery were his words. With cunning and compassion Lincoln altered the dynamics of the Civil War with the stroke of his pen...which was exactly his intent.
Although Lincoln did personally believe that slavery was evil, his decision to actually abolish it was a war tactic.
We can see at least two proofs of this. First, there is the fact that Lincoln wrote (in a famous letter to Horace Greeley) that he would be willing to save the Union by freeing all of the slaves, or some of the slaves, or none of the slaves, whichever was needed. This shows that his main goal was to save the Union, not to help the slaves. Second, there is the fact that Lincoln delayed issuing the Emancipation Proclamation until he thought it would help the war effort. He did not issue it right away but instead waited until the North had won a battle so that it would make the North look strong rather than desperate.
We cannot know for sure what Lincoln's motives were, but these two pieces of evidence imply to me that freeing the slaves was a tactical action, not a humanitarian one.