Asking your ex to leave was the right thing to do. You shouldn't feel bad about that at all. But it is also entirely understandable that you do feel bad and confused about it. Perhaps it would be easier to think of it like "this has happened because DH was the kind of person who had to be asked to leave" rather than "this happened because I asked DH to leave". It's obviously not ideal that you had to ask him to leave, but you wouldn't have done it unless it had to be done.
Continuing to handle that situation well is about ensuring that you minimise the effects of that split on your children. That is more likely to be done by accepting that split will affect them, and working on minimising/counterbalancing that effect, not by hoping it won't affect them. It will. Quicker you accept that and get on with dealign with it the better. It is entirely understandable to hope that there has bee no fallout and that everything is ok, but that doesn't make it accurate. But not asking him to leave would have affected them too. It's better that you did ask him to leave. But that, unfortunately is not the end of what you will have to deal with.
The split also brings with it knock-on effects for your children that need to be accepted and dealt with as they are. That doesn't detract from the fact that asking your ex to leave was the right thing to do. The fact that children will be upset when their parents split up is pretty much unavoidable. How you show you are a good parent in this is how much you help them to deal with that (even if that means getting other people to directly help rather than yourself), not by getting caught up in guilt and going back on the original decision, or trying to pretend that everything is fine.
It's not about taking your ex back, it's about making sure you get the right help for your DS that takes his experience of the situation into account (whether or no that experience is correct in it's assessment of the rights and wrongs of things). You need to accept how he feels and start from there, not try to make it into a situation that it isn't.
You won't be able to hide your feelings of guilt, hurt etc around the split from DS. That is entirely normal and understandable, and unavoidable. But you need to understand that he will sense those feelings, and he will clam up because he doesn't want to hurt you/he is generally fearful or upset just now. Getting in a tailspin about what people are thinking about you and are you doing the right thing only underlines how emotionally enmeshed you are in this- he needs someone neutral. He needs someone to talk to who is not as invested in the situation- so he doesn't have to worry about how what he says will affect them, but is able to speak freely.
I wouldn't rush too quickly into "DS has picked up DH's bad behaviour because I didn't shield him enough/act quick enough either". Don't assume the behaviour is about copying your ex. It could just be acting up due to the general situation. Don't tar a child with "you take after your father". It could be that, but it could also be anxiety, missing his dad, setting the boundaries, confusions over standards of behaviour when he sees various adults saying one thing but doing another, just plain unhappiness and stress. All of which are pretty normal reactions to this from an 8 year old. He reactions might not be "correct" or "right" but they are valid, and they are his reactions.