Is Django now an outlaw? Are there any witnesses left?

There were certainly plenty of witnesses to the event. The household help who were allowed to escape still continue to be slaves of the Candies. IMO, both the housekeeper and Candie's mistress, Sheba (?), would, perhaps with a little persuasion, be happy to cooperate with any investigators.

And investigation there will be, considering the nature of the incident and the personage involved. Moreover, considering Dr.Schulz's body at Candieland as well as the trail left behind at Greenville, plenty of people are aware of Django's existence. Schulz and Django have also had previous dealings with lawmen during their bounty hunting; Django's name and identification would therefore also be widely available.

Whether Django will be branded an outlaw depends on the testimony of the Candieland employees. But he'll certainly be wanted for questioning.