I believe Schultz was a pragmatic man, and he had never been that closely involved with slavery before, and he was obviously disgusted by the situation and of course he didn't like Candie and he had probably decided that that man should die since the incident with the dogs.
So my explanation is that killing Candie was a spontanious descision to shorten the process. He basically got upset by Candie to the point where he decided to do it there and then, without waiting and going through the hassle of concocting another plan. He knew it was wrong, and he knew he would die, that was why he apologized, but he just didn't want to wait any longer, because he probably thought it would be hard to get to him again once they had left.
“It appeared that there had even been demonstrations to thank Big Brother for raising the chocolate ration to twenty grams a week. And only yesterday, he reflected, it had been announced that the ration was to be reduced to twenty grams a week. Was it possible that they could swallow that, after only twenty-four hours? Yes, they swallowed it.”
- George Orwell, ‘1984’
