D.K.
DS is 5, not 6 yet so I can only tell you what works for us - YMMV. Things that help.
1. Assume positive intent - she is doing the best she can for who she is right now (not who you want her to be or who she will be next year)
2. Showing DS respect - getting down at his level to talk to him. Looking at him when talking and asking whether he has heard. Telling him things politely (if someone at work talked to me like that would I be ok with it? if not, it's not respectful).
3. Separating what he says from how he says it. If he 'I want my toy now and I will yell if I don't get it' I try (not always easy) to determine whether his request is reasonable. If so my response would be to model how he should ask for the toy - 'so what you are saying is mommy, may I please have my toy now" and then give it to him. Threats are a normal developmental stage - they are learning the power of words - we just identify them as such - 'DS, that is a threat, when you say that it makes me want to not help you, when you say, mommy please may I, then I want to help'.