I knew (?)
Shouldn't be there "I know" ?

Annotation:
This means that before I did it, I knew that I shouldn't do it, but even with that knowledge, I went ahead and did it.

Thank you for the explanation!

I cannot avoid the impression that ‘I knew I shouldn’t have done it’ is not quite logical if ‘I knew I shouldn’t do it’ is what it is supposed to mean: The deed was not yet done, and I knew I shouldn’t do it, but I did it anyway.

Is what you wrote colloquial (even apart from the ‘anyways’)? I cannot believe that is standard, but then, I still have much to learn. ☺

I would agree with Pfirsichbaeumchen here. If you read this sentence quickly, you understand the meaning and it sounds natural, but if you take a second look, it does, in fact, look like something is wrong in the logic department.

It's a quite common expression.

Try this google search to find other similar examples.

https://www.google.com/search?q...+have+done+it"

It's often used with only the first half of the sentence, with the second half being implied.

@CK:

This doesn't make it correct. However, I agree that it's common, sounds natural, and would be used by native speakers.

(is there something like a "dubious logic" tag for cases like this?)

I'm a bit late to the party, but I have my doubts about "anyways".

"I did it anyways" is not uncommon.

https://www.google.com/search?q...ys%22+-tatoeba

My googling nevertheless tells me it's informal and colloquial.

I would say "anyway", but "anyways" to me is still valid

"anyways" may be more common in AE than in BE.

This might be one of those cases where it doesn't follow the book 100%, but it's naturally used so much that there's no point in making a fuss about it.
One of those things that textbooks can't really prepare a learner for.

descriptive grammar vs prescriptive grammar

https://www.google.com/search?q...iptive+grammar

Or, if you prefer videos, you can limit this search to YouTube.

https://www.google.com/search?n...mAFmF_1W05AAAA

David Brown's explanation is one that I refer people to the most often.
https://youtu.be/ugf_yCelFE4

Caroline Heycock's explanation is also very good.
https://youtu.be/xq5g82Kx8c4