See part 1
This capital sin is named envy which is related to avarice and it’s the tendency to be sad by another’s good. And many time, the sadness actually causes the person who is experiencing it to desire that the other person be deprived of the particular good in question.
We know that we are being envious when we feel uncomfortable when we hear someone else being praised. Perhaps you’ll remember yourself in a group of people at a dinner or get-together where someone is saying something good about someone else and if you become uncomfortable by hearing this, that is a sign that you are being attacked by envy. A sure sign that this is happening is being in this situation and someone criticizes the person being praised with “oh yeah but” (if there really [isn’t] any good reason to criticize). When a person is envious, they try to bring the other person down from those who are praising him or her by criticizing him or her.
Envy by its very nature is a mortal sin because it is directly opposed to charity (which would require us to rejoice in the good of others). St. Thomas Aquinas tells us that the greater the good we envy, the graver our sin is. This means therefore that when we are envious of someone’s spiritual goods, then this is the most grievous kind of sin of envy. When, for example, we are envious because somebody else has a good prayer life, or someone else does spiritual reading, or someone else is very charitable and does lots of works of charity for others — then we have committed a very grievous sin.
So how do we remedy envy in our life? On the one hand, we need to crush these feelings as soon as we notice them. So as soon as you get that feeling in your heart; of feeling uncomfortable because someone else is being praised, or you hear that someone else is being praised and in your mind you start thinking of all the bad things this person is — immediately crush it, because that is envy trying to come forward; trying to rear its ugly head. The other thing we need to do is practice emulation.
We emulate the saints. The saints are held up because of their virtue and their good works. We try to imitate them. Well, we could do the same with the goods that others have: if someone is an honest person, if someone is loyal, we can strive to imitate them in these good things. That is what emulation is: a desire to imitate to equal and, if possible, to surpass the good qualities of others.
Jealousy is […] defined to be a sorrow which one entertains at another’s well-being because of a view that one’s own excellence is in consequence lessened. Its distinctive malice comes from the opposition it implies to the supreme virtue of charity. The law of love constrains us to rejoice rather than to be distressed at the good fortune of our neighbour. Besides, such an attitude is a direct contradiction of the spirit of solidarity which ought to characterize the human race and, especially, the members of the Christian community.
The envious man tortures himself without cause, morbidly holding as he does, the success of another to constitute an evil for himself. The sin, in so far as it defies the great precept of charity, is in general grievous, although on account of the trifling matter involved, as well as because of the lack of deliberation, it is often reputed to be venial.
Jealousy is most evil when one repines at another’s spiritual good. It is then said to be a sin against the Holy Ghost. It is likewise called a capital sin because of the other vices it begets. Among its progeny St. Thomas (II-II:36) enumerates hatred, detraction, rejoicing over the misfortunes of one’s fellow, and whispering.
Regret at another’s success is not always jealousy. The motive has to be scrutinized. If, for instance, I feel sorrow at the news of another’s promotion or rise to wealth, either because I know that he does not deserve his accession of good fortune, or because I have founded reason to fear he will use it to injure me or others, my attitude, provided that there is no excess in my sentiment, is entirely rational. Then, too, it may happen that I do not, properly speaking, begrudge my neighbour his happier condition, but simply am grieved that I have not imitated him. Thus if the subject-matter be praiseworthy, I shall be not jealous but rather laudably emulous.
Source: see part 1