In my understaning of 'masscares', essentially none of the opposing combatants are left alive. Not so at Fort Pillow. The Fort was given every opportunity to surrender and it elected to not do so. Fort Pillow doesn't even appear to be a 'no quarter' siege - a type of attack which has been used on ocassion by most civilized countries.
The fact that there were so many African-American soldiers in the Fort seems to have some bearing on the discussion of a massacre at Fort Donelson. This perspective of the assault seems to have no merit as 20% of the African-Americans were taken alive as prisoners. No particular race was exempt from being shot, including both commander and the subsequent Union commander of the Fort.
The prisoner exchange was brought to a halt by the Union forces (Grant & Lincoln). They demanded that all enlisted escaped slaves be treated as POWs as well. The South refused to accept this condition. These captivess would be returned to the plantation from whence they had escaped and not be made available for prisoner exchange.