Aradia always had a matesprit crush on Sollux. Ever since they first talked when they were small wigglers, she had those feelings. It was only a few sweeps later that the two actually became matesprits.
Aradia and Sollux spent a lot of time together. Many dates, many night outs, many movie nights, each and every one of them fun for both of them. They were friends from a very young age, so it was easy for them to come up with ideas to do things together.
Everything wasn't completely dandy and happy, they did fight a few times. In the end though, they both made amends and go back to happiness and glee. It was all happy in till that one day.
A hint of gloom already masked the day, and Aradia noticed that before she was going to get Vriska for what she did to Tavros. She felt like she had to get her back, as if Vriska wasn't going to seek revenge for it.
It was really quick to be honest, at least up in till the very end. She went unconscious, but the last time she was able to open her beautiful eyes, she could see the pain Sollux was in. His tears litter her face, and he says he's sorry over and over again.
Aradia raises a gentle hand up to his face, stopping him from saying his apologizes for the thousandth time. She moves her mouth, but nothing came out of her mouth as she tries to speak to him. Sollux puts one of his hands atop hers, still crying his yellow tears. She lastly says one sentence, and that sentence is "I will always love you."
Sollux cries over her dead body, so terrible that he killed her. He did understand that he was being mind controlled by Vriska, but the fact that it was him who killed Aradia, not Vriska, pains him greatly.
He slowly got over it though, as Aradia's ghost could see. Subconsciously, she watched him live his life, only talking with him from time to time. She remembered what she said to him before her death, and somewhere deep in her mind and soul, those words were still true.