What was the meaning of Schultz's flashback of dogs?

You have slightly muddled the occurrence of events. Schultz has a flashback of the dog-mauling scene not right after Candie ups Broomhilda's(Django's wife) price, but later when the deal is being put to paper. In the violent moment where Candie threatens to bash Hilda's skull with a hammer, Schultz agrees to the raised price in a breath.

Later, when Dr. Schultz has a moment all by himself, sitting in the arm-chair, does he begin to experience the flash-backs of the hideous episode(dog-mauling). I believe he had those visions as a result of the trauma he had to undergo while witnessing the scene. The doctor was indeed a violent man, but not cruel and certainly didn't agree with slavery or Candie's methods.

Excerpt:

Calvin Candie: Your boss looks a little green around the gills.

Django: He just ain't used to seein' a man ripped apart by dogs is all.

The climatic scene that you mention, was the repercussion or the end result of this flashback. Candie managed to antagonize Dr. Schultz so much over Broomhilda's deal and later(remember the insistence to shake hands) that he decided to pull the trigger.

EDIT: IMO, Schultz never considered the possibility (however remote or substantial) that Candie was going to kill them all. Rather than this being an act of premeditated self defense, this was more of an aggravated assault by Schultz not only against Candie's physical form but also against his misplaced ideals (read slavery, penchant for killing etc.).

In fact my answer is very similar to KeyBrd Basher's but I'm still going to post my views:

I for myself wouldn't say that with this flashback he realized Candie was going to kill them all (which I'm not even sure was the case). I would rather say that this flashback just raised his anger over Candie's cruelty and increased his moral aversion against him, which then in the end led to him shooting Candie when he was angered even further to shake hands with such a man.

The point here is Schultz's aversion against Candie and slavery in general. In fact for me a much more important question was the deeper reason for this aversion. Of course slavery was a bad thing but at that time I think it still was something special to be as strongly against slavery as Schultz was (but maybe there wasn't much to it and he was just supposed to be a "good guy").

So even if at first Schultz seemed a bit violent or emotionless to Django (when he demanded to kill that one criminal in front of his son and urging Django to play that role of a black slave trader), this shifted when they finally entered Django's world and he realized that there were still things he hadn't seen yet. In the end he didn't expect the journey to be that traumatic and thought it would be a nice adventure helping Siegfried to get back Brunhilde and at the same time making an active statement against slavery by helping his "new black friend".

So in this flashback he recapitulated what evil he was really standing against there. When he was then finally urged to shake hands with a man he hated so much, he just couldn't resist.

The flashback to be mindful of here is not of Schlutz seeing the dogs mauling at D'Artagnan, but the point of time where he asks Django to kill Smitty Bacall in the presence of his son and when Django confronts Schlutz on their way to Candyland with Candie in relation to Bacall"s killing.

This stirs up his moral sense when he witnesses D'Artagnan's killing which subsequently makes him kill Candie...