Almost certainly not as evil as the Tudor "spin doctor" William Shakespeare would have us believe.
Much of our understanding of Richard's evil doing comes from the Tudor period. Henry Tudor's claim to the throne was nowhere near as strong as Richard's so he enforced his own claim over Richard's by battle, killing Richard at the Battle of Bosworth. So Henry Tudor and his descendants had their own reasons for manipulating public opinion against Richard to justify their own position. Some of the Tudor claims, such as he spent 2 years in his mother's womb are clearly ridiculous. Because there is no claim amongst his contemporaries to support such accusations, we can also be quite sure that the Tudor claims that he poisoned his wife and son are based on nothing more than malicious propaganda, and that they in fact probably died of tuberculosis.
The record shows, in fact, that Richard was a wise and just ruler, courageous in battle, energetic in his endeavours and before the death of his brother (Edward IV) a loyal brother.
A loyal brother, a king who introduced a bail system in court to make the legal system fairer, a king who abolished benevolences (compulsory gifts that had to be made to the monarch). We must wonder if these really are the actions of an evil king.
That he moved rapidly upon his brother's death to take the two princes (his nephews) into his care and to usurp the throne is beyond doubt. But that action does not neccesarily make him evil. We can find motives other than a selfish desire for personal power for his actions.
By moving quickly to remove the princes from their escort party as they were travelling from Ludlow to London, Richard removed them from the influence of his brother's "in-laws" the Woodville family who themselves were using the princes for their own dynastic ambitions.
By taking them to the Tower of London, and accomodating them in the Royal Apartments there, it could well be that Richard was keeping them in a place of safety, not a place of imprisonment. Richard was after all their protector. And there are at least two other candidates for the heinous crime of their murder, although we cannot even be certain if they were murdered.
So, the one undisputable act that he committed that makes us question his morality is the fact that he usurped the throne from his nephew. What would drive a seemingly loyal brother, and just ruler to commit such an act.
To look for a possible motive we must consider the unenlightened times in which he was living.
It was brought to his attention that the the two princes were illegitimate. His brother (Edward IV) married Elizabeth Woodville who was the mother of the two boys. However at the time of his marriage it seems that Edward was betrothed to Lady Eleanor Butler. In those days a betrothal was as strong a commitment as marriage, and thus made the sons of his marriage to Elizabeth illegitimate. An illegitimate king would have been seen as unthinkable as this would bring down the wrath of God on the kingdom. You could argue then that as a loyal English nobleman he could not contemplate even the slightest possibility of one of the princes becoming King. Against all his natural inhibitions as their uncle and protector, Richard may have felt it was his inescapable duty to England to prevent the prince becoming King by usurping the throne himself.
To muddy the water's even further, recent documents found in Rouen cathedral, which was headquarters for the English army during their campaiging in France during Edward IV's reign, indicate that Edward IV was away on a campaign at the time that his first son (Edward V) was conceived. This would seem to suggest that the young prince was not even of royal blood, but was in fact (it is suggested) the son of an archer in Edward's army.
So, the one indisputable act of his that could suggest he was an evil man, in fact could have a noble explanation, given the unenlightened times he was living in.
It is quite possible that by keeping the princes, firstly in the Tower, and perhaps later by moving them to his own lands in the North of England, he was keeping them safely out of harms way.
If this hypothesis is correct, then the princes may well have lived out the rest of their lives in anonymity. However if Henry Tudor (VII) happened to discover them after his victory at Bosworth, he would without a doubt have wanted them murdered, and there is one of the prime suspects for their murder, if Richard is innocent and the princes were actually murdered.
There are a few "ifs" in this hypothesis, but there is probably as much (if not more) evidence for this hypothosis as there is for the Tudor version. And even if this hypothesis is correct, it does not rule out the possibility that Richard still committed the murder of his nephews, but his motive far from being personal were for what he perceived as being for the good of the realm.