While it is apparently the vogue among scholars to depreciate Thomas Jefferson's ambiguous stance towards slavery, there are many effective actions he took to ameliorate or abolish slavery. First of all, history seems to have forgotten his anti-slavery language in the original draft of the Declaration of Independence. It condemns the introduction of slavery to America by the British King as one of the grievances listed. Here is the actual text:
king of Britain] has waged cruel war against human nature itself,
violating it’s most sacred rights of life & liberty in the persons
of a distant people who never offended him, captivating & carrying
them into slavery in another hemisphere, or to incur miserable death in
their transportation thither. this piratical warfare, the opprobrium of infidel
powers, is the warfare of the CHRISTIAN king of Great Britain.
determined to keep open a market where MEN should be bought & sold,
he has prostituted his negative for suppressing every legislative
attempt to prohibit or to restrain this execrable commerce: and that
this assemblage of horrors might want no fact of distinguished die, he
is now exciting those very people to rise in arms among us, and to
purchase that liberty of which he has deprived them, by murdering the people upon whom he also obtruded them; thus paying off former crimes committed against the liberties of one people, with crimes which he urges them to commit against the lives of another.
Now that is pretty strong language. Unfortunately, due to the opposition of the South Carolina and Georgia delegations, it was removed.
Another concrete action he took to stop or slow down the spread of slavery was his drafting of the 1784 Territorial Governance Act which would have outlawed slavery in all of the territories between the Appalachian mountains and the Mississippi River. However, this act was defeated by one vote. It did give rise to the passage of the Northwest Ordinance of 1787, which did outlaw slavery in Ohio and states to the north of it.
Also as President he urged Congress to abolish the slave trade which imported slaves to the US, and in 1807 this was accomplished. So even though Jefferson was a slave owner and may have shared the contemporary mindset about the inferiority of blacks, there is solid evidence he also considered slavery a great evil and did something about it.