The sentiment expressed in the quote "Jealous is an ugly thing" from the book "Sentinel" by Jennifer L. Armentrout reflects the negative impact that jealousy can have on individuals and relationships. This emotion often leads to destructive behavior and can cloud one’s judgment, causing rifts between friends and loved ones. It highlights the importance of self-awareness and emotional management to prevent jealousy from taking root in one's life.

In the context of the book, this theme may resonate through character interactions and conflicts that arise from envy. Jealousy not only harms those who experience it but also affects those around them, leading to tension and unhappiness. Armentrout's exploration of these dynamics serves as a reminder of the ugliness of jealousy and encourages readers to foster positivity and understanding instead.