Objection 1. It would seem that one is guilty of murder through killing someone by chance. For we read (Genesis 4:23-24) that Lamech slew a man in mistake for a wild beast [The text of the Bible does not say so, but this was the Jewish traditional commentary on Genesis 4:23, and that he was accounted guilty of murder. Therefore one incurs the guilt of murder through killing a man by chance.
Objection 2. Further, it is written (Exodus 21:22): "If... one strike a woman with child, and she miscarry indeed... if her death ensue thereupon, he shall render life for life." Yet this may happen without any intention of causing her death. Therefore one is guilty of murder through killing someone by chance.
Objection 3. Further, the Decretals [Dist. 1 contain several canons prescribing penalties for unintentional homicide. Now penalty is not due save for guilt. Therefore he who kills a man by chance, incurs the guilt of murder.
On the contrary, Augustine says to Publicola (Ep. xlvii): "When we do a thing for a good and lawful purpose, if thereby we unintentionally cause harm to anyone, it should by no means be imputed to us." Now it sometimes happens by chance that a person is killed as a result of something done for a good purpose. Therefore the person who did it is not accounted guilty.
I answer that, According to the Philosopher (Phys. ii, 6) "chance is a cause that acts beside one's intention." Hence chance happenings, strictly speaking, are neither intended nor voluntary. And since every sin is voluntary, according to Augustine (De Vera Relig. xiv) it follows that chance happenings, as such, are not sins.
Nevertheless it happens that what is not actually and directly voluntary and intended, is voluntary and intended accidentally, according as that which removes an obstacle is called an accidental cause. Wherefore he who does not remove something whence homicide results whereas he ought to remove it, is in a sense guilty of voluntary homicide. This happens in two ways: first when a man causes another's death through occupying himself with unlawful things which he ought to avoid: secondly, when he does not take sufficient care. Hence, according to jurists, if a man pursue a lawful occupation and take due care, the result being that a person loses his life, he is not guilty of that person's death: whereas if he be occupied with something unlawful, or even with something lawful, but without due care, he does not escape being guilty of murder, if his action results in someone's death.
Reply to Objection 1. Lamech did not take sufficient care to avoid taking a man's life: and so he was not excused from being guilty of homicide.
Reply to Objection 2. He that strikes a woman with child does something unlawful: wherefore if there results the death either of the woman or of the animated fetus, he will not be excused from homicide, especially seeing that death is the natural result of such a blow.
Reply to Objection 3. According to the canons a penalty, is inflicted on those who cause death unintentionally, through doing something unlawful, or failing to take sufficient care.
Source: Summa Theologica (New Advent)