Oops,
From what you have said it does sound like ASD. However like twocutedarlings said it is important to get a proper diagnosis as there are other conditions which can show with similar symptoms.
However you may benefit looking at some of the ways of dealing with such children and trying them to see if they help.
To be honest with what you have said the response to the new school does not surprise me. It took him a long time to get settled into the old school and become comfortable. He would have expected the new one to be the same, even possibly with the people he knew at school being there. With ASD/AS this would have thrown him and he would not cope.
The following is my view only, and based on observations of my daughter and having worked with other autistic children for a few months. Meltdowns come about when the child feels they have lost all control over what is happening around them or to them. It is a mechanism which tells people 'LEAVE ME ALONE!!!' and this is important. The child needs time to calm down and regain their composure. They cannot do this if there is too much stimulus going on, including touching or someone talking to them.
As hard as it is, and at times it may just not be possible, you need to let your son calm down on their own terms. This means simply ignoring him. When he is ready he will rejoin the family. This has improved our family life a great deal.
He will probably take what you say literally, so you do need to be careful with criticism. Again with teasing, it is something to be wary off, he is unlikely to realise it is a 'bit of fun' and may well see it as a continuation of bullying from school if he is teased there.
Yes it is scary to watch, but unless it results in self harming behaviour ie bashing head on something there is nothing to be afraid of. Easier said then done, but it does get better with practice.
You do need to get a handle on what he is saying about wanting to kill his sibling, and see you dead. It is extremely unlikely that he means these things and they are said as a way of hitting back and getting a reaction. I found five sheets of paper where my daughter had written all sort of things about me, none of which were complimentary, and in general wishing I was gone but they could keep the money I was earning. My reaction was one of delight. I had finally found a way my daughter was comfortable with sharing her emotions and speak out about what was happening to her. It didn't bother me because I recognised it was written by an angry child who was upset. I did speak to her and arranged for her to have a diary with the rule that no one was allowed to read it unless she chose to show it. That way others could not get upset by reading what she had written and misunderstanding it.
She used to plot with her year 7 learning mentor how they would kill me and some of the teachers, there were some realy bizarre methods. I just told them it wouldn't work because my army of ninja worms would protect me. She still talks about them.
Please don't dismiss what you are going through as petty. Yes there are those going through worse, there are some who are greatful they don't have to deal with what you have to! It is your reality and no less painful or significant for that. There are times we all need support and help.