According to Aristotle, whatever exists BY NATURE is as good as it can possibly be. But a "nature" is a specific class, by definition, rather than a single entity, while "people" are individual entities or individual beings by definition.
Aristotle also points out that nothing which exists by NATURE can form a habit contrary to its own nature. For example, he argues, no one can train a stone to fly because it falls to earth by nature. Similarly, no one can train fire to fall, rather than rise, because fire rises by nature. Herbivores can't be trained to be flesh eaters (carnivores), nor can carnivores, be trained to digest plant cellulose. So, by nature, certain animals are flesh eaters and other animals, by nature, are plant eaters. In sum, there is nothing good or evil BY NATURE.
However if you were eaten by a flesh eater, or trampled to death by a herd of buffalo who were running toward green grass and fresh water (the buffalo being hungry and thirsty, but not for you), you would suffer an evil in both cases. You would be a "good" to the meat eating carnivore and neither good, nor evil [just an "accident"], to the running buffalo who caused you to suffer an evil [ injury followed by death ] when they trampled you. So good and evil, which ARE contraries, can be accidental [from the Latin word "accidit" which means "to happen"] or natural goods. You are a natural good [meat] to a carnivore and may suffer an accidental evil by being either eaten by a carnivore or trampled by a herd of herbivores.
But you are not talking about either "accidental evils" or "natural goods" when you ask the question, requote:- Are people, by nature, good or evil?
You are, instead, asking whether or not people DO GOOD ACTS or DO EVIL ACTS, by "nature". The answer is neither. We do not do good acts by nature. We do not do evil acts by nature. Instead, once again, according to Aristotle, quote
ARISTOTLE:
Neither, by nature, then, nor contrary to nature do the virtues [good habits KB] arise in us; rather we are adapted, by nature, to receive them and are made perfect by HABIT. [Nicomachean Ethics; BK II, Ch. 1., 1103a lines 24 - 25]
So the 3 controlling ideas for this topic must be the ideas expressed by means of the words:
(1) nature, (2) good and (3) evil.
Aristotle is, by far, the best philosopher on the topics of nature, good and evil. Quote
ARISTOTLE:
"Again of all things that come to us by nature we first acquire the potentiality and later exhibit the activity (this is plain in the case of the senses; for it was not by often seeing and often hearing that we got these senses, but on the contrary we had them [i.e. we had the senses of seeing and hearing by nature KB] before we used them and did not come to have them by using them); but the virtues [a.k.a. good habits KB] we get by first exercizing them, as also happens in the cases of the arts as well. For the things we have to learn before we can do them, we learn by doing them, e.g. men become builders by building and lyre players by playing the lyre; so, too, we become just by doing just acts, temperate by doing temperate acts, brave by doing brave acts ...
... Again it is from the same causes and by the same means that every virtue is both produced and destroyed, and similarly every art; for it is from playing the lyre that both good and bad ("evil") lyre-players are produced. And the corresponding statement is true of builders and of all the rest [of artists]; men will be good or bad builders as a result of building well or badly. For if this were not so, there would have been no need of a teacher, but all men would have been born GOOD or BAD at their craft. This, then, is the case with virtues also; by doing the ACTS that we do in our transactions with other men we become JUST or UNJUST ... brave or cowardly ... temperate or intemperate, etc. [and, in general, good or evil KB; Nicomachean Ethics BK II, Ch. 1.; 1103a line 26 - ... - 1103b line 20 ]
So doing good ACTIONS or doing evil ACTIONS is not a matter of "nature", but rather HABIT, according to Aristotle. The fact that Aristotle calls GOOD HABITS by the name of VIRTUES [albeit in translation to English from Greek] and BAD or EVIL HABITS by the name of VICES shouldn't make a difference to your understanding that it is human HABITS [repeated acts] which may be good [virtuous or virtues] or evil [vicious or vices] rather than human nature being good or evil. It is not human nature that is good or evil, but, rather, human habits/conduct which are/is good or evil, according to the habits we form.
Kevin