Search Results: he likes me but he has a girlfriend

Imagine a situation where:
-Older Brother (30): In a committed relationship for 3-4 years, living with his girlfriend.
-Younger Brother (27): Had a tumultuous, on-again/off-again relationship with a woman (25) over the past year.
-The Ex-Girlfriend (25): Struggled with the relationship due to repeated cheating, emotional conflicts, and mistreatment by the younger brother

When the Ex-Girlfriend First Reached Out to the Older Brother, He Was Supportive and Neutral → Initially, he listened, gave neutral advice, and didn’t seem to have a strong opinion about her or the relationship.

-However, After That, the Older Brother Became Harsh & Judgmental → Over text, he started expressing strong dislike toward the ex-girlfriend, saying she was manipulative and toxic.
-The Older Brother Has Reached Out Multiple Times to Confront the Ex-Girlfriend → Instead of ignoring the situation, he actively engaged with her in texts, being dismissive and harshly critical of her role in the relationship.
-The Ex-Girlfriend Tried to Explain Her Side → She reached out to the older brother to show him the full picture, explaining that she had been hurt, cheated on, and mistreated. However, the older brother ignored this perspective and continued blaming her entirely for the toxic cycle.
-Even After the Breakup, the Older Brother Still Feels Strongly About the Situation → Despite the fact that the younger brother and the ex-girlfriend have broken up, the older brother continues to speak negatively about her and express strong feelings of resentment toward her.

What do you believe are the primary reasons for the older brother’s intense and harsh behavior toward his younger brother’s ex-girlfriend?