Some people were punished for less.
Less sounds like a noun here.
Now I wonder, can it be substituted with "the lesser", as "the lesser" is also a noun. "The lesser of two evils".
If it can, what would be the difference?
If it can't, what's the reason for that?
This is an interesting question! Yes, some dictionaries define less as a noun, and it can be used somewhat like one, as in "People have been convicted for less", but it also is not exactly like a noun, because you can't refer to "a less" or "the less". I would really consider it an adjective, because "People have been convicted for less" functions like an abbreviated version of "People have been convicted for less serious acts."
You cannot use the lesser in this sentence, though, because the lesser is explicitly the most minor of the (usually two) things being compared, and there is nothing here being compared. You would use it like "Of the two charges brought against him, he was convicted only of the lesser".
No, "lesser" can not be substituted for "less" in your example sentence, because they have different meanings. "Less" normally means a smaller amount. According to M-W, "lesser" means:
of less size, quality, degree, or significance : of lower status
For example, it is common to choose the "lesser" of two evils, but it would be quite unusual to choose the "less" of two evils. (It is possible to say that, but the meaning would be a bit different.)
This might seem confusing, and in practice, it may not always be easy to distinguish between "lesser" and "less". Nevertheless, they are not exact synonyms. If it helps, you can think of "lesser" as the opposite of "greater" and "less" as the opposite of "more".
