It's not something I put a lot of thought into. Cain would have been under the old law, even BEFORE the law of Abraham, and he was punished immediately and specifically for his sin. So, I don't really see any reason to say that repentance would be impossible, at least in theory. There are, however, some other circumstances surrounding Cain that could pose other issues, but for the sake of *THIS* discussion, they aren't important. Cain had as much chance as Adam or Eve. They sinned, and he sinned. They were punished, and he was punished.
As for Judas...it's a bit harder to make any real discussion about this. We don't know the exact circumstances surrounding his death exactly, nor his motives. Though, I do remember a verse that describes how Satan entered into Judas before the betrayal.
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