Cain was overcome by the sin of envy and it resulted in murder. While you may not be struggling with envy on the level that Cain did envy can be spiritually deadly to your soul.
The Catechism defines envy as the Capital Sin that refers to the sadness at the sight of another’s goods and the immoderate desire to acquire them for oneself even unjustly. When it wishes grave harm on another it is mortal.
Envy can be a really ugly sin if you think about it. The person who is ensnared by the sin of envy is sad when good things happen to others. How awful and lonely a soul that is. Some people reading this will recognize envy very well for they are greatly given over to its demands while some people may need to really search their souls in order to root out the sin of envy.
I can honestly say that some of ugliest stains that have been on my soul have been from the scourge of envy. I have been so envious of the spiritual lives of others. At times it has been a constant struggle. I am ashamed to say the sin of envy has been so ravaging to my soul that I have publicly criticized the spiritual lives of those who I envied. I am still sad I let it get so bad.
St. Augustine called Envy: “The diabolical Sin.” If St. Augustine took it so seriously then we must as well. It is impossible to achieve intimacy with God when we are envious of the spiritual lives of others. This would be similar to trying to achieve intimacy with your own wife while being envious of the relationship another man has with his wife.
How do we combat Envy? Simply by growing in the love of others and taking sinful thoughts of envy captive before they run wild. Pray each and every day for the mind and the eyes of Christ Jesus our Lord.
What rust is to iron, what moths are to wool, what termites are to wood, that envy is to the soul: the assassination of brotherly love.
Bishop Fulton J. Sheen
Whenever you envy your neighbor, you give demons a place to rest.
St. Ephraem the Syrian
And if you do not do well, sin is lurking at the door; its desire is for you, but you must master it.