Objection 1. It would seem that wife-murder is not an impediment to marriage. For adultery is more directly opposed to marriage than murder is. Now adultery is not an impediment to marriage. Neither therefore is wife-murder.
Objection 2. Further, it is a more grievous sin to kill one's mother than one's wife, for it is never lawful to strike one's mother, whereas it is sometimes lawful to strike one's wife. But matricide is not an impediment to marriage. Neither therefore is wife-murder.
Objection 3. Further, it is a greater sin for a man to kill another man's wife on account of adultery than to kill his own wife, inasmuch as he has less motive and is less concerned with her correction. But he who kills another man's wife is not hindered from marrying. Neither therefore is he who kills his own wife.
Objection 4. Further, if the cause be removed, the effect is removed. But the sin of murder can be removed by repentance. Therefore the consequent impediment to marriage can be removed also: and consequently it would seem that after he has done penance he is not forbidden to marry.
On the contrary, A canon (caus. xxxiii, qu. ii, can. Interfectores) says: "The slayers of their own wives must be brought back to penance, and they are absolutely forbidden to marry." Further, in whatsoever a man sins, in that same must he be punished. But he who kills his wife sins against marriage. Therefore he must be punished by being deprived of marriage.
I answer that, By the Church's decree wife-murder is an impediment to marriage. Sometimes however it forbids the contracting of marriage without voiding the contract, when to wit the husband kills his wife on account of adultery or even through hatred; nevertheless if there be fear lest he should prove incontinent, he may be dispensed by the Church so as to marry lawfully. Sometimes it also voids the contract, as when a man kills his wife in order to marry her with whom he has committed adultery, for then the law declares him simply unfit to marry her, so that if he actually marry her his marriage is void. He is not however hereby rendered simply unfit by law in relation to other women: wherefore if he should have married another, although he sin by disobeying the Church's ordinance, the marriage is nevertheless not voided for this reason.
Reply to Objection 1. Murder and adultery in certain cases forbid the contracting of marriage and void the contract, as we say here in regard to wife-murder, and shall say further on (Sent. iv, 62, 2) in regard to adultery. We may also reply that wife-murder is contrary to the substance of wedlock, whereas adultery is contrary to the good of fidelity due to marriage. Hence adultery is not more opposed to marriage than wife-murder, and the argument is based on a false premiss.
Reply to Objection 2. Simply speaking it is a more grievous sin to kill one's mother than one's wife, as also more opposed to nature, since a man reveres his mother naturally. Consequently he is less inclined to matricide and more prone to wife-murder; and it is to repress this proneness that the Church has forbidden marriage to the man who has murdered his wife.
Reply to Objection 3. Such a man does not sin against marriage as he does who kills his own wife; wherefore the comparison fails.
Reply to Objection 4. It does not follow that because guilt has been remitted therefore the entire punishment is remitted, as evidenced by irregularity. For repentance does not restore a man to his former dignity, although it can restore him to his former state of grace, as stated above (38, 1, ad 3).
Source: Summa Theologica (New Advent)