Heart of Darkness

Are human beings inherently good or evil?

As a species; No.
As individuals; Potentially.

It's arguable whether a person can be truly evil if they think they're doing the things they do for right/justified reasons or whether they have to know they're doing things that are bad/wrong but choose to do them anyway.

In either case exists the potential to be instinctively more aggressive, less compassionate, more self centered, outright sociopathic and similar based on heredity.
There also exists the potential to be more compassionate, cautious, group focused and forward thinking due to heredity.
Those sets of features could be considered the precursors towards typically accepted definitions of behaviors that are good/evil and without any external influences necessitating them being overridden. To answer part b), yes it's possible to change.

Even the most caring and loving and naturally genial person can become a cold blooded killer if they believe it righteous. To an external observer, particularly their 'vicitms' they would meet the criteria of evil; Right down to knowing it's morally wrong to kill but doing it anyway because they feel their imperative supercedes their morality. It's also possible for them to be broken by an event to the point where they no longer have the capacity to care about adherence to a moral compass.

A person predisposed towards aggression and violence but with a clear understanding of the ramifications of their impulses and a decent sense of self preservation can moderate their behavior sufficiently enough to not be openly evil on a notable scale, or channel it into other activities, but the underlying urges and feelings wouldn't necessarily be gone, so it depends on whether you consider evil in the act or the thought. They can still be a seething ball of hate while living within the facade of societal normalcy.

For these reasons I ultimately find the concept of good and evil meaningless.

People are people. They can do bad things. They can do good things. They will be driven by internal urges and external influences. They may act in ways they don't wish. They may want to do things they can't. Almost no one lives as their own ideal self because almost no life is given situations in which ideal answers are possible.