The word used in Ps 37:20 is רְשָׁעִים (rᵉšāʿı̂m). This is an adjective (lexical form רָשָׁע, rāšāʿ, "wicked") used as a substantive (i.e. a noun, "the wicked ones").
There are two terms for the abstract "wickedness" derived from the same root (רשע, r-š-ʿ): רִשְׁעָה (rišʿāh), a feminine noun spelled differently from the word of interest, and a masculine segolate noun רֶ֫שַׁע (rešaʿ) which indeed has the same consonants as the word of interest.
There are a few reasons I see that the consonantal רשעים can not be from the abstract noun רֶ֫שַׁע (rešaʿ) here:
The word רְשָׁעִים (rĕšāʿı̂m) is plural. Although certain abstract nouns are plural, רֶ֫שַׁע (rešaʿ) is not one of them. It does not appear anywhere in the HB in the plural.1 In contrast, the adjective is usually plural.2
The verb is plural. Although certain abstract nouns are plural, the verbal predicate of a singular idea is generally singular (Gesenius-Kautzsch-Cowley § 145h).
The word רְשָׁעִים (rĕšāʿı̂m) is in parallel with both תְמִימִם (tĕmı̂mim, "the blameless [ones]", antithetically) and אֹיְבֵי יְהוָה (ʾōyĕbê yhwh, "the enemies of the LORD", synonymously). These most naturally all refer to concrete entities, people.
1. One may doubt that others have been correctly understood/translated. In most cases, there is really no question. See, for one example among many, Isa 13:11 where רְשָׁעִים must represent the ones punished, not the abstraction "wickedness."
2. I did not count how many of the 249 instances are substantive; it's common.
