Who Will Be Saved?
John 3:16 For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.
John 3:17 For God sent not his Son into the world to condemn the world; but that the world through him might be saved.
John 3:18 He that believeth on him is not condemned: but he that believeth not is condemned already, because he hath not believed in the name of the only begotten Son of God.
John 3:19 And this is the condemnation, that light is come into the world, and men loved darkness rather than light, because their deeds were evil.
John 3:20 For every one that doeth evil hateth the light, neither cometh to the light, lest his deeds should be reproved.
John 3:21 But he that doeth truth cometh to the light, that his deeds may be made manifest, that they are wrought in God.
John 3:18 states that the ONLY path to salvation is belief in (and acceptance of) Jesus Christ.
John 3:20 and 3:21 states that one that doeth evil rejects the light (Jesus Christ), but he that doeth truth comes to the light (Jesus Christ).
So it kind of works out such that all relatively good people will be saved, by their choice. Not because of their good works, but because they will be drawn to the light (Jesus Christ). Conversely all truly evil people will not be saved, by their choice. Not because of the evil they do, but because they will reject the light (Jesus Christ).
When John 3:20 says “one that doeth” evil, “that doeth” is the Greek word “prasso” which means “to practice, that is to perform repeatedly or habitually”. Think of it this way, before you were saved did you always intend to do that which is evil? Most times didn’t you intend to do what is good? Well a truly evil person that John is referring to only intends to do what is evil.
prassō (G4238)
A primary verb; to “practise”, that is, perform repeatedly or habitually (thus differing from G4160, which properly refers to a single act); by implication to execute, accomplish, etc.; specifically to collect (dues), fare (personally): - commit, deeds, do, exact, keep, require, use arts.
Here is an example that may help to understand the difference between the “one that doeth evil” and “he that doeth truth” in John 3:20 and 3:21.
Frank and John both need a job. There are two different job openings requiring many workers and paying the same amount with the same benefits. One job is at a concentration camp torturing and killing innocent Jews. The other is at a homeless shelter feeding and housing the poor and needy.
Frank takes the job at the concentration camp. John takes the job at the homeless shelter. So which one of these doeth evil and which one doeth truth? Their choice shows what is in their hearts. While John does not always do good, that is what he is drawn to. While Frank may not always do evil, that is what he is drawn to.
Do you think my example of what Frank chose is far fetched? Well we have a perfect example of that with the concentration camps in Germany when Hitler was in power.
Yes, there are people that are just that evil!
Goodboy 
You may also want to read something else I posted.