Generally speaking, we might distinguish the right of death from power over life by saying that the former is a negative kind of power and the latter is a positive kind of power. During the age of absolute monarchies (Louis XIV of France being the most famous example), the king was thought of as the embodiment of the state. Any harm done to the state was metaphorically seen as harm done to the king himself. The punishment for crimes was consequently seen as the king's (or the state's) vengeance on this transgressor. If a man were caught stealing, that man was stealing from the king, and the king would have the right to punish him accordingly.
The power exercised by absolute monarchies took the form of "thou shalt not." Citizens were forbidden from doing certain things that would be of harm to the state, and consequently to the king, and if they broke these prohibitions they would be punished. They weren't expected to do anything in particular; instead, there were limitations (laws, taxes, military service) placed on their freedom. Outside these limitations, people could live as they pleased.
Absolute monarchy came to be replaced by bourgeois society and modern capitalism. This society exercised a positive kind of power: power over life. The emphasis here was not on what one couldn't do, or on the limitations placed on people's freedoms. Rather, the emphasis was on what people should do, or how their freedom ought to manifest itself. This was the age of liberalism and republicanism, where slogans of human liberty were at the forefront of revolutions in France and America. Foucault would suggest that these revolutions did not free the people from an oppressive power so much as they replaced one form of power with another. These revolutions overthrew absolutist regimes that seemed to have little concern for the lives of its citizens. They were replaced by regimes that had a deep interest in the lives of its citizens: so much so that how people lived and how people ought to live became a matter of public importance. "Thou shalt not" was replaced with "thou shalt."
There are two reasons why it is problematic to make moral judgments regarding this comparison. First, it is difficult to separate out the good and the bad. For instance, it might seem good that bourgeois society has taken a stronger interest in the life and health of its people. At the same time, this "good" is balanced out by the fact that this interest has been very invasive. One's private life has become a matter of public interest.
The second reason moral judgments are so difficult is because Foucault is discussing the powers that have shaped who we are. We do not have a distanced enough perspective on the matter to make a balanced judgment. Any judgment for or against bio-power would essentially be a judgment for or against our contemporary way of life.
Obviously, there is not a clean break between one form of power and another, and Foucault does not claim there is. Further, these forms of power manifest themselves in many different ways. For instance, during the Cold War, both the United States and the Soviet Union exercised a kind of bio-power: both countries had a vested interest in the education, health, economic productivity, fertility, and so on, of its populace. However, these two countries had very different ideas of what was right for its people, and set about securing this way of life in very different ways.