why do feel so let down when somoene we love decided to do the unthinkable, you cheat and dont feel remorseful about it still stating they had to do i

Because they have gone against the social values that you were so carefully ingrained with growing up.

It's a well known fact that a child's most informative years are before the age of seven. Going against those values can only cause guilt.

Not learning those values before the age of seven can affect anyone in a negative way. Often they are unempathic as well as lack remorse.

The expectance of those values is as well ingrained before the age of seven. We expect those values to be upheld. We really don't like change. Even though life is full of changes in everything.

The easiest path is the well worn path. And forbid it if that path becomes blocked. We are no longer within what we expect. So the conflict on what to do starts.