Regulus has had a crush on James for years. He can't even remember when it started; it felt as though it was always there. They became friends and James would talk about Lily and she was now friends with them.
He hates her for something she didn't do. She is kind to him and does everything with grace. She didn't deserve any of this. So, he ignored the feelings, shoving them to the deepest place in him.
Watching them bashfully flirt but not notice the other is flirting. The blushing, the awkward body language. The touches that last a little too long, the desperate touches, eyes constantly searching for a way to do so.
It makes Regulus sick of bitterness like he was forced to drink straight lemon juice into his mouth.
He hates himself for it. He hates his emotions for doing this for him, for making him fall in love and making him jealous and bitter. He hates James for being in his life, and unknowingly stabbing his heart repeatedly. He hates Lily for being his desire.
But they don't deserve any of it. His feelings that he has doesn't make them the villain. It is his fault, his problem alone.
When they got together, he felt the most betrayed. But they didn't know. So it is his fault. It is his stupid fucking brain doing this shit to him and then to them.
They don't deserve this. He does. All the shit I used to say about muggle-borns and even said to Lily, and yet she still cares for him and even forgives him even though she didn't have to.
This was his punishment. His purgatory. Seeing the one he loves romantically gets with one he loves platonically.
They are happy so it is fine for him to go through hell.
He is just a jealous, bitter man. As his mother would put it.
And this time she is right.