Thaumaturgical magic is as moral as electricity and as ethical as sociology. While we may fervently hope that scientists, technicians, engineers and researchers will attain and use their knowledge and skills in a way that will benefit us (and which we are thus likely to consider "ethical and moral"), we have no way of enforcing our moral and ethical standards upon the laws of physics, genetics, sociology or mathematics. Therefore, each culture decides what it considers appropriate behavior by those who wield these powers, then tries to enforce those boundaries.
The commonest forms of social control for wielders of all kinds of power are religion, spirituality, and philosophy. When it's magical power that people are worried about being handled "properly," theurgical systems of magic are created which build into themselves artificial restraints of a moral and ethical nature. Thus, contrary to a currently popular opinion in some occult circles, "evil" spells do not automatically rebound from an innocent victim back to the caster three times as strong, unless the caster believes they will. That belief may be based on naivete, in that the caster believes whatever he has been taught or has read in books, or it may be based on a deliberate oath or geas (a type of magical restriction) that he has consciously taken upon himself as a spiritual decision, perhaps during an initiation or ordination.
What is true is that a dedication to "Evil For Evil's Sake" is a cardinal sign of a psychotic who will eventually slit his own throat through committing some grossly stupid blunder. Similarly, while purity and innocence do not guarantee protection from malevolent magical attack, in some game systems intense religious piety may provide some divine protection.
No psychologically healthy person (or other intelligent critter) engages in activity of any sort without having some idea, however vague, of whether he, his fellows, and/or his society at large would approve or disapprove, that is, would consider his actions good or evil. It is a sad truth that all too many evildoers in history have thought themselves good, and been thought good by others. What one game character may consider an evil act (such as using magic to steal gold from a wealthy man) another character might consider perfectly reasonable and good (if it's to feed a starving family, for example). Each game and each group of players will have to make individual decisions about how to define "good" or "evil" - yet another reason why representatives of the Religious Reich are horrified by gaming.
The "Lawful vs. Chaotic" political alignment system invented early by D&D players (probably taken originally from Michael Moorcock's Eternal Champion cycle of fantasy and science fiction stories) is far easier to define. But as players and referees have noticed, considering the "lawful" Nazis and the "chaotic" Robin Hood, these terms cannot be equated with moral ones. But it can be amusing to watch how the real-world political attitudes of players and referees get involved in automatic assumptions about the morality of order and anarchy.
The bottom line about magical motives in gaming, however is this:
"Evil" magic is done in pretty much the same ways that "good" magic is done - only the psychological and spiritual/ethical flavors are different, though these will affect such issues as what sort of spirits will help or hinder the magic as well as reactions by other characters.
But let me close this topic by repeating the Law of Positive Attraction, that "like attracts like." A dedication to "demonic" pursuits tends to attract "demonic" energies (many of whom are aligned "Hungry") and repeated destructive acts usually wind up attracting destruction (that's why you never meet a genuinely happy and successful diabolist). This is especially true in the psychic realms, where physically weak "victims" may have sneaky and unexpected ways of lashing back at their unwary attackers (unlike most victims of physical violence). Or to put it another way, "those who live by the wand, often die by the wand."
Any of the methods used by magicians to raise mana in magical ceremonies can be used (and often are) by Clerics and worshippers to raise mana for religious rites. Breathing exercises, meditation, potions, singing, chanting, dancing, and sex are all used, depending upon the nature of the deities involved.
There are different motives, often mixed, for making sacrifices: Apotropaic sacrifices are offerings made to dangerous or Evil spirits in an effort to encourage them to go away. Propitiatory sacrifices are offerings made to gain or regain the favor of a spirit one may have offended. Thanksgiving sacrifices are simply a way of showing the spirit worshiped that his blessings are appreciated. Supportive sacrifices are offerings made to strengthen the spirit worshiped or to express one's love for him. The whole essence of a sacrifice ("holy offering") is that the humans making it decide that they will do without the energy in the object or being that is being sacrificed, so that the spirits worshiped may have it.
The idea that fruits and vegetables, which are often sacrificed to spirits, have any mana may seem strange, unless you have seen Kirilian photos of plants that have been injured. Huge sprays of energy, which many researchers think is (or is related to) mana, suddenly gush forth out of the damaged plants. Apparently some worshippers manage to focus this energy and to send it along with the mana of their prayers to their deities. Naturally, animals and humans have a great deal of mana, and animal sacrifice is quite common around the world.
Some deities like animal or human blood (which can certainly make you emotional while donating it) but aren't interested in total sacrifices (if you kill a person or animal, it won't be able to donate blood next month). If this sounds too gruesome, consider the Zulus who lived (until a few decades ago) on a diet composed primarily of milk and blood from their cattle, which they both milked and bled regularly. Blood sacrifices are messy but they are not always the sign of an "Evil" deity. Hunting gods, for example, often expect the first blood and meat from newly-slain game.
Human sacrifice is not frequent, even in those cultures that practice it (except for the Aztecs, who were - according to one theory - short of meat protein in the Valley of Mexico and long on prisoners of war). Most tribes simply don't have large enough gene pools to make human sacrifice on a large scale practical. Instead, it is most commonly used as a form of capital punishment. The amount of mana to be gotten out of human sacrifices is large (basically all of the person's MPs) but the process is risky unless the victims are willing, since they may always choose to spend their dying mana surge on a curse against the sacrificers! Nonetheless, the assertion that the only Mages or Clerics performing human sacrifices are "Evil" ones, while plausible in most circumstances, is not always true.
For that matter, Yahweh, the God of Israel, had blood sacrifices (doves, for example) made to Him right up to the destruction of His temple in Jerusalem by the Romans - not to mention the millions of human sacrifices made to the One True Godª under His various names for the last four thousand years.
But most deities, especially the creative ones, prefer other bodily liquids in sacrifice (such as milk, orgasmic fluids, or the sweat from hard work) or else such items as wine, fruits, flowers, expensive incenses, etc.
Nonphysical sacrifices include the mana generated from abstinence and/or fasting, the keeping or breaking of food or other taboos, hard work, artistic creation, etc.
The whole point of a sacrifice of any sort is that the worshippers are "feeding" the deity with their human energies (mana) in addition to any energies the sacrifice may have on its own.
This is why gold, silver and jewels (which have little if any mana of their own) are so often sacrificed. It's not just that the Clerics want lots of money for the local temple (though that is sometimes part of it), it's also a matter of people getting very emotional when they give up something of high financial and/or artistic value. If you sacrifice a consecrated sword or golden torc to a deity, you are giving the stored mana in it to the deity - so breaking the sword or torc and throwing it into a pit or a lake can make perfect sense, either as part of a ritual of mana release or as a way to ensure that no one will ever desecrate the object.