A. We cannot consider worthless people who...
B. It cannot be considered worthless people who...
C. People who... cannot be considered worthless.
I would like to say the sentences like A. My biggest problem is due to B, as I wonder if it is correct.
Any comment would be appreciated
We need a more complete context to examine this usage in detail, so let's look at...
1: We cannot consider worthless people who get drunk on New Year's Eve.
...which has two different possible meanings...
1a: We cannot consider [as] worthless those people who get drunk on New Year's Eve.
(They are not worthless, and we should not label them as such.)1b: We cannot consider those worthless people who get drunk on New Year's Eve.
(They are worthless, and we should not take them into account in our present deliberations.)
In speech, sense 1a would normally be unambiguously conveyed by stressing the word worthless, but italicizing it in the written form wouldn't necessarily have the same effect anyway. That's because there are two potential "sub-nuances"...
1a1: ...worthless isn't the best possible word here (following text may suggest better alternatives).
1b1: ...[they are not] worthless is a simple refutation (they do have some merit).
Partly because of these potential semantic ambiguities, OP's example A is thus at least "problematic" in the written form (plus it's a somewhat stylized/stilted word order). B is simply ungrammatical. But C is perfectly natural and unambiguous.
The only way I can think of to introduce "existential it" in OP's context would be something like...
2: It would be wrong to consider people who get drunk on New Year's Eve [as / to be] worthless.
That's to say it references either the specific action consider people who [blah blah] OR "judgement" as a general concept. It can't be used in such contexts to directly refer to the [worthless] people themselves.
