This is a complicated question with no clear answer; if one is to determine whether humans are good or evil, you must first define the two terms. Since everyone has a different definition of good and evil it is impossible to do so. For something to be "defined" everyone must agree on what it means. Morality as a whole is a difficult subject, let's go with stealing for example in my own sense of morality it is okay to steal from the rich and those who have gained their wealth have through ill means but not the average person or those who struggle. Many people would disagree with my stance but some people would agree, similarly, the death penalty is a subject that has been debated time and time again some say it's no different than murder and it makes you no different than the people who are sentenced, I will avoid voicing my stance as that is too political.
I for one consider myself a logical egoist, I'll put myself first, those I love second, neutrals third, and enemies forth. I'll help those I love even if it puts me at risk, help strangers as long as it does not go against my interests, and help my enemies if I have something to gain. I won't cross my bottom line and won't tolerate those who do but everything else is okay in my book. Does that make me evil or good? I don't know as I've never really identified with the terms even if I do have my own definition.
The world is not black and white nor is it shades of gray, it's an array of infinite colors, so many that you could never scratch the surface in a single lifetime, or many for that matter. In my opinion, the most beautiful thing about humanity is that array, everyone is so different, we may seem similar on the surface, there may be some things that most people agree is right or wrong, but in the end, the many slight differences culminate to create a large one.
So to return to your original question "are humans good or evil", going on the assumption that my definition of good and evil is 100% correct, which I highly doubt, I'd like to believe they have the potential to be both. Within the world of infinite possibilities that we live in who is to say such a thing as "pure good" and "pure evil" exist at the same time why can't people be a mix. Just as the world has infinite possibilities humans have infinite potential, just because someone does something I consider bad one day doesn't mean they lack the potential to do good as well.
Continuing to your third question, while one human may not make much impact as they are but a single spec of dust on the pages of time, together they can accomplish greatness. To reject one's humanity is to reject those possibilities, should one reject their humanity not only are they rejecting themselves but they are rejecting everything that they could do. Life is short one must grasp it by the reins and do with it what they will. Regardless of whether their mission ended in success or failure, regardless of whether their mission was right or wrong the fact that they choose to do something instead of wasting their life brooding over being born human makes them far more valuable in the long run than those who reject humanity. After all many "evil" people have done things that proved useful in the long run.
To summarize my long and perhaps contradictory post, good and evil cannot be clearly defined, thus as a whole good and evil cannot exist, however, if it is merely opinions you're looking for you should have no problem finding some just be prepared to have a very diverse pool of results. One should not reject their humanity as it squanders their potential after all as I've said many times humans are a race of infinite potential that includes the good, the evil, and everything inbetween.