From the Catechism of the Catholic Church:
2258 “Human life is sacred because from its beginning it involves the creative action of God and it remains for ever in a special relationship with the Creator, who is its sole end. God alone is the Lord of life from its beginning until its end: no one can under any circumstance claim for himself the right directly to destroy an innocent human being.”
2260 The covenant between God and mankind is interwoven with reminders of God’s gift of human life and man’s murderous violence. The Old Testament always considered blood a sacred sign of life. This teaching remains necessary for all time.
2261 Scripture specifies the prohibition contained in the fifth commandment: “Do not slay the innocent and the righteous.” The deliberate murder of an innocent person is gravely contrary to the dignity of the human being, to the golden rule, and to the holiness of the Creator. The law forbidding it is universally valid: it obliges each and everyone, always and everywhere.
2268 The fifth commandment forbids direct and intentional killing as gravely sinful. The murderer and those who cooperate voluntarily in murder commit a sin that cries out to heaven for vengeance.
Infanticide, fratricide, parricide, and the murder of a spouse are especially grave crimes by reason of the natural bonds which they break. Concern for eugenics or public health cannot justify any murder, even if commanded by public authority.
2270 Human life must be respected and protected absolutely from the moment of conception. From the first moment of his existence, a human being must be recognized as having the rights of a person – among which is the inviolable right of every innocent being to life.
2271 Since the first century the Church has affirmed the moral evil of every procured abortion. This teaching has not changed and remains unchangeable. Direct abortion, that is to say, abortion willed either as an end or a means, is gravely contrary to the moral law.
You shall not kill the embryo by abortion and shall not cause the newborn to perish. – Didache 2:2
God, the Lord of life, has entrusted to men the noble mission of safeguarding life, and men must carry it out in a manner worthy of themselves. Life must be protected with the utmost care from the moment of conception: abortion and infanticide are abominable crimes. – Gaudium et Spes – Pope Paul VI, 1965
2272 Formal cooperation in an abortion constitutes a grave offense. The Church attaches the canonical penalty of excommunication to this crime against human life. “A person who procures a completed abortion incurs excommunication latae sententiae,” “by the very commission of the offense,” and subject to the conditions provided by Canon Law. The Church does not thereby intend to restrict the scope of mercy. Rather, she makes clear the gravity of the crime committed, the irreparable harm done to the innocent who is put to death, as well as to the parents and the whole of society.
2273 The inalienable right to life of every innocent human individual is a constitutive element of a civil society and its legislation:
The inalienable rights of the person must be recognized and respected by civil society and the political authority. These human rights depend neither on single individuals nor on parents; nor do they represent a concession made by society and the state; they belong to human nature and are inherent in the person by virtue of the creative act from which the person took his origin. Among such fundamental rights one should mention in this regard every human being’s right to life and physical integrity from the moment of conception until death.
The moment a positive law deprives a category of human beings of the protection which civil legislation ought to accord them, the state is denying the equality of all before the law. When the state does not place its power at the service of the rights of each citizen, and in particular of the more vulnerable, the very foundations of a state based on law are undermined. . . . As a consequence of the respect and protection which must be ensured for the unborn child from the moment of conception, the law must provide appropriate penal sanctions for every deliberate violation of the child’s rights.” – Donum Vitae, III
2274 Since it must be treated from conception as a person, the embryo must be defended in its integrity, cared for, and healed, as far as possible, like any other human being.
Prenatal diagnosis is morally licit, “if it respects the life and integrity of the embryo and the human fetus and is directed toward its safe guarding or healing as an individual. . . . It is gravely opposed to the moral law when this is done with the thought of possibly inducing an abortion, depending upon the results: a diagnosis must not be the equivalent of a death sentence.” – Donum Vitae, I
2275 “One must hold as licit procedures carried out on the human embryo which respect the life and integrity of the embryo and do not involve disproportionate risks for it, but are directed toward its healing the improvement of its condition of health, or its individual survival.”
It is immoral to produce human embryos intended for exploitation as disposable biological material.
Certain attempts to influence chromosomic or genetic inheritance are not therapeutic but are aimed at producing human beings selected according to sex or other predetermined qualities. Such manipulations are contrary to the personal dignity of the human being and his integrity and identity” which are unique and unrepeatable. Donum Vitae I
2276 Those whose lives are diminished or weakened deserve special respect. Sick or handicapped persons should be helped to lead lives as normal as possible.
2277 Whatever its motives and means, direct euthanasia consists in putting an end to the lives of handicapped, sick, or dying persons. It is morally unacceptable.