Hi
my DS is 6.5 (Y2 at school) and it just... so defiant.
We've had violence (lashing out when angry) issues on and off since my marriage ended two years ago, but it had really settled down since easter.
Even when not being violent he is extremely defiant at home. It isn't all the time, and a lot of the time he is a lovely kind, caring boy. But when he gets these moods - normally when I say its time to do something he doesn't want to do - he's awful.
I just don't know what to do when he's like that! An eg below:
Me - DS in two minutes it will be time for bed so your kindle will need to go off.
DS - OK
Me - Right DS, time for bed. Turn your kindle off please (this is just an example and will relate to ANYTHING he is doing when he doesn't want to do something else)
DS - Ignores me
Me - DS, I'm going to count to 10 and if your kindle isn't off, I'll be taking it away
DS - Ignores me
Me - right , you won't be playing that tomorrow now DS.
DS - Screaming "You're so mean!"
Me - Come on, up to bed (ignoring the silliness)
DS - NO
Me - DS, you can either go upstairs nicely like a big boy, or I can take you like a little boy if you can't behave
DS - Ignores me
Me -
DS - Starts screeching, might hit me, probably cry
Me -
Those nights will often end with him hitting his door or something in anger.
Tonight he was being very unkind to his older sister so I told him it wasn't nice and to apologise, which he ignored and carried on. I told him he had 3 seconds to apologise or he would be off to bed early (only 10 mins before bedtime anyway).. he ignored, so I went to pick him up calmly to take him up... he started hitting and screaming at me. Then ran off upstairs.
I followed him up and calmly explained that he had the chance to apologise but refused so he had to go to bed, I asked if he was going to get changed nicely or if he'd have to go to bed in his day clothes - he said he wasn't changing, so I turned to leave him to calm down.
At which point he whacked me over the head with a hexbug set! I admit I did the wrong thing here and lost it at him but that bloody hurt! I screamed, and shouted at him that it was a horrible thing to do - walked out shutting his door, and promptly burst into tears for 5 mins on my bed 
It was mostly shock. I went in once I'd calmed down but he was still being awful so I left again and went back another 5 mins later when he was sobbing that he was sorry and he didn't mean to hurt me!
There are times when he will simply refuse to do something and I have no idea at all how to make him. EG getting in the car to go somewhere - I've tried to go pick him up and put him in but he'll run off. I refuse to pay hide and seek to get him to do as he's told. I've tried the walk away thing and it eventually works sometimes, but long after it should do.
We talk about who is in charge and who makes the rules - and when he's in a defiant mood he'll say he is and no one makes his rules. Tonight he agreed that I am in charge and make the rules.
We have strategies for his anger which do help when that happens (which are why the violence is much rarer now), and we talk about how home is a place where you feel loved and safe, and I asked if he feels loved and safe (he does) and if he thought I did after that (he didn't think so). He agreed it wasn't fair etc - but the after bit is always easy, he's a clever kid - HOW do I deal with the defiance and enforce the structure and rules he needs?!