Objection 1. It would seem that a man ought not to be debarred from receiving Orders on account of homicide. Because our Orders originated with the office of the Levites, as stated in the previous Distinction (Sent. iv, D, 24). But the Levites consecrated their hands by shedding the blood of their brethren (Exodus 32:29). Therefore neither should anyone in the New Testament be debarred from receiving Orders on account of the shedding of blood.
Objection 2. Further, no one should be debarred from a sacrament on account of an act of virtue. Now blood is sometimes shed for justice' sake, for instance by a judge; and he who has the office would sin if he did not shed it. Therefore he is not hindered on that account from receiving Orders.
Objection 3. Further, punishment is not due save for a fault. Now sometimes a person commits homicide without fault, for instance by defending himself, or again by mishap. Therefore he ought not to incur the punishment of irregularity.
On the contrary, Against this there are many canonical statutes [Cap. Miror; cap. Clericum; cap. De his Cler., dist. 1; cap. Continebatur, De homic. volunt.], as also the custom of the Church.
I answer that, All the Orders bear a relation to the sacrament of the Eucharist, which is the sacrament of the peace vouchsafed to us by the shedding of Christ's blood. And since homicide is most opposed to peace, and those who slay are conformed to Christ's slayers rather than to Christ slain, to whom all the ministers of the aforesaid sacrament ought to be conformed, it follows that it is unlawful, although not invalid, for homicides to be raised to Orders.
Reply to Objection 1. The Old Law inflicted the punishment of blood, whereas the New Law does not. Hence the comparison fails between the ministers of the Old Testament and those of the New, which is a sweet yoke and a light burden (Matthew 11:30).
Reply to Objection 2. Irregularity is incurred not only on account of sin, but chiefly on account of a person being unfit to administer the sacrament of the Eucharist. Hence the judge and all who take part with him in a cause of blood, are irregular, because the shedding of blood is unbecoming to the ministers of that sacrament.
Reply to Objection 3. No one does a thing without being the cause thereof, and in man this is something voluntary. Hence he who by mishap slays a man without knowing that it is a man, is not called a homicide, nor does he incur irregularity (unless he was occupying himself in some unlawful manner, or failed to take sufficient care, since in this case the slaying becomes somewhat voluntary). But this is not because he is not in fault, since irregularity is incurred even without fault. Wherefore even he who in a particular case slays a man in self-defense without committing a sin, is none the less irregular
Source: Summa Theologica (New Advent)