1 John 5:19 wickedness or wicked one?

I don't think that Conjurske is completely accurate here. My understanding of Greek words are that they are either masculine, feminine, or neuter in their root and when they are inflected (indicating case and number) the gender of the word is not affected. (Note: I think there may be few rare exception words that are placed in more than one category.)

Therefore, the same masculine adjective poneros (Strong's #4190) in verse 18 is also masculine in verse 19; this is confirmed by Robertson in his Word Pictures of the New Testament. This should not be confused by the fact that Greek singular dative forms of the nominative or dative case have the same ending (spelling).

The Greek adjective is used in a nominative sense in verse 18 and preceded by the definite article ("o"), literally meaning together something like "the evil" or "the wicked". A corresponding word for "one" is not present in the Greek. Perhaps implied by the context, the word "one" is supplied by the translators (yet the KJV does not put it in italics). Just because the word is considered masculine for grammatical purposes, it does not follow that it must indicate the biological gender of the real-world object (obviously, many objects for which there are words are inanimate or do not reproduce). In this case, there are no contextual clues as to the physical sexuality of the subject; from this passage alone even a female entity cannot be ruled out. Therefore, "one", "thing", or another 'gender neutral' singular term could be inserted by the translators.

The same grammatically masculine Greek word is also preceded by a dative form of the definite article in verse 19, and collectively these words could approximate "to the evil" or "for the wicked" in English (again, there is no corresponding Greek word for "one" or "thing" here). Definite articles are not always present in Greek (but typically more frequently than in English), nor is it always neccessary or desireable to translate them into English each and every time they are present.

I think if the Greek words in verse 18 are to be rendered something like "the Wicked One" (designating a personal agent) then it certainly should follow that a corresponding indication be made in verse 19; it seems that the author is exhibiting a parallelism between "we" that are of the Holy God and those ("the whole world") that belong to an Evil One.

Some of the more literal versions --

We know that we are of God, and the whole world lieth in the evil one. (American Standard Version)

we have known that of God we are, and the whole world in the evil doth lie; (Young Literal Translation)

We know that we are of God, and the whole world lies in the wicked [one]. (Darby)

We are aware that we are of God, and the whole world is lying in the wicked one. (Concordant Literal Version) ​