Who does Harriet Beecher Stowe, author of Uncle Tom's Cabin, blame for slavery?

This is open to debate, as you're talking about the
background motivations of an author in the 1850s.  The novel itself was written in
response to the passage of the Fugitive Slave Act, requiring Northerners to aid the
recapture of runaway slaves.  In this sense, she blames
lawmakers. 

On another level, Stowe blames slaveowners,
some of whom are portrayed as kind and paternalistic in the novel.  To her, slaveowning
is an absolute moral wrong, regardless of how kind or cruel an owner
is.

Lastly, I think she places blame on those in the North
who tolerate slavery, or refuse to take up the abolitionist cause.  I think she is most
angry with this crowd, particularly northern Chrisitians.