I have also known a DC like this!
You have been given a really hard time op, but I can appreciate your frustration. When you are trying to teach your own DC boundaries and how we must not hurt others. another comes along and causes havoc with no boundaries.
Infact two dc I have known like this, both extremely violent. One I only saw at toddler groups, I joined late and the other mums didn't seem to react except to comfort their battered DC ( Dc used to swing chairs at their heads, stand on trampoline so could get good kick into toddlers going by, took a running jump at Dc head on the ground.....etc), so I did think perhaps there was something else going there....that they all knew but I did not...so I kept my DC away from that one DC - steering away as much as possible in as subtle a way as possible.
The other one, my goodness.....the DM just seemed so blind and oblivious, I saw that evil flash in the eye - and long thought out ways to harm, I saw a crying mother - cradling her baby - whom this DC had just kicked...and the DC Dm did nothing except get upset herself.
She dotes soo much and is sooo fussy about so many things, really anal and yet her DC has caused havoc time and time and time again, at nursery, parties, play dates...play centers!
Personally I used to watch my DC like a hawk at that age, and even if she occidentally bumped into someone or took a toy, I made her apologise, give the toy back etc. When we went through a difficult phase I just had follow through on threats and leave.
I just wouldn't want my dc making other DC as frightened as mine was sometimes when being hurt by the others.