DS2 - will be 2 and a half in January - and I am.at a loss with him.
He is incredibly bright and articulate (I know everyone says that but really - he's starting to read, can have long complex conversations, generally has the memory of an elephant) and physically very ahead of himself too - he climbs like a monkey and can ride a bike. But he is so naughty. If things don't go the way he wants he screams and screams, thrashes about and hits (really hard!). He also yells the 2yo equivalent of obscenities: (eg 'you're a stinky little bumface mummy'). It is definitely done to hurt rather than just out of frustration - one strategy we tried for a while on someone's advice was simply ignoring bad behaviour - and if he didn't get a rise from hitting me he would escalate to pinching, scratching or even biting me, or pulling my hair, or even go to attack his older brother or the cat instead (both of whom he adores) because he knew I'd have to react to stop him before he did that.
He gets masses of attention, I only work 3 days a week so he's with me the rest of the time and usually with DH one of the other days as he's a shift worker. He's with a childminder he loves the other 2-3 days a week and generally behaves beautifully there. She regularly comments on how kind and empathetic he is to the other children. He has a very good friendship with her daughter and another little girl who're both a year older than him - he plays proper make believe games with them, looks forward to seeing them, and talks about them when he's not there. He can be incredibly kind, empathetic and loving to us and his older brother too, but then it just seems to vanish.
He's never seen any violence at home. We don't smack or do physical punishments (though I do sometimes have to restrain him to stop him hurting himself or others) - his older brother went through a tiny phase of hitting when he was about one and it was dealt with with a few shocked faces and 'oh we don't do that!' He's never hit since and has never been anything but kind to his little brother.
I'm totally out of my depth with this - it's been going on for over a year now. We thought the terrible 2s had just hit early as he seems to do everything early, but they've never stopped. The bigger he is the harder it is to manage. He can be utterly delightful but then it's like a switch flips. I've tried (each strategy for a reasonable length of time, not just at random) being shocked and hurt, cuddling and reassuring him that we love him while explaining to him it's not ok, getting really angry and shouting 
, time-outs, removal of toys/ treats, ignoring bad behaviour and massively rewarding good behaviour, giving alternative outlets (eg a cushion to hit when he's angry, or words to say, like 'I'm feeling so cross at the moment')... We make sure we're on the same page so he's not getting mixed messages, and it feels like we've tried everything.
Am I just being ridiculous worrying this much about this, he's only 2 and will outgrow it? Or is this a serious issue we need to solve now before he grows up into a serial killer...?
Any ideas?