Context:

So in this mod, Pink (real name Pinku), used to be a cruel, heartless impostor that worked at the Impostor Company with some of the other characters (like Black, Green, Maroon, etc). But one day she met Yellow (real name Dillaw) when spying on the Crewmate Company. She fell in love and realized she had a choice to make. She could either choose LOVE or PURPOSE.

If she chooses love, (which is the canon choice), she betrays the company and becomes Yellow's lover, only for the company to send Black and White after her in an attempt to kill her. Later on, Yellow is killed by White.

If she chooses purpose, (which is what is seen in this song), she becomes a psychopathic impostor and kills Yellow, severing her ties with him. She also kills William (Grey) and Mitori (Green) and they can be seen in the background

tldr: Pink used to be heartless, but she met Yellow which led her to choose between being nice or being a psycho. In this alternate timeline, she chose psycho.