If I walked away from somebody who made me feel like absolute shit, am I the one to blame?

You should have explained how you felt, to give them some closure.

But other than that, not really

Nothing's your fault; you should always walk away from people who make you feel like shit because they're bad for you and you should surround yourself with positive people. I think you should be straight with them now though and tell them why you stopped hanging out with them, for closure.

You have no obligation to be nice to someone who does nothing but hurt you.
Now, if that person does do nice things for you once in a while, he/she may be trying to be nice but is having a hard time with it. You should do what you think is best, but try to think about it before you do it.

If your relationship with someone is destructive then you have every right to walk away. But I think you do owe them an explanation. Just so that you put it all behind you and move on. Even if they respond badly to what you say, you shouldn't feel bad about it because you have done your best to resolve the situation.

You should tell them the truth.... maybe they feel like its a slap in the face to them. They are wondering. Dont worry you are not the one at fault... it would though be a good idea to tell them why you have changed sides.?? One thing for sure you do not need to feel guilty because that will just eat you. its human nature to change so its fine.

hell to the fucking no

No way! Why should you stay around people who make you feel like shit? That's only a stress booster not to mention it would be super annoying to have to face someone who always shuts you down. But if they ask you that again, explain why. Hopefully, they understand. And they might stop doing whatever it is that's making you feel like shit. Sometimes, you have to point out a flaw.