Francis Scott Key's Wikipedia page has two interesting paragraphs pertaining to his view of slavery. Under 'Slavery and the American Colonization Society" it reads: "
Key publicly criticized slavery's cruelties, so much that after his death a newspaper editorial stated "So actively hostile was he to the peculiar institution that he was called 'The Nigger Lawyer' .... because he often volunteered to defend the downtrodden sons and daughters of Africa. Mr. Key convinced me that slavery was wrong—radically wrong."In June 1842, Key attended the funeral of William Costin, a free, mixed race resident who had challenged Washington's surety bond laws."
The article notes he also founded the American Colonization Society, which is described on its Wikipedia page as a "coalition... who supported abolition of slavery and believed blacks would face better chances for freedom in Africa than in the United States."
However, in the very next section the page discusses his opposition to abolitionism. These two paragraphs seem to contradict each other without providing any nuance, and I am interested in understanding what specifically Key's views were.
I don't study Key himself and so don't know a lot about his political life, but what's being described in your quotes is an actual political position and probably the most popular one adverse to slavery for the first half of the nineteenth century. So yes, you can be anti-abolitionist and antislavery simultaneously but it relies on distinctions in nineteenth century discourse that aren't very familiar today unless you study them. I do that. :)
The very short version is that antislavery people support some form of gradual, usually compensated emancipation enacted through the ordinary political process and often coupled with colonization to get rid of the freedpeople because white Americans don't want that many black people around without slavery to "control" them. They are opposed by abolitionists who are usually for more drastic action, which would be immediate and uncompensated as well as conclude with black Americans still resident in the country.
The longer version is something I wrote a little while ago, which I think covers all the high points. If anything's unclear or you have further questions, I'm happy to help.