'sang off key in my chorus cause it wasn't yours' means he didn't care about her or her problems cause it wasnt his (he sang off key in her chorus cause he didnt care enough to put the effort to sing it correctly cause it wasn't his, so when you apply it to life it means he cared only about himself not her)
"sang off key in my chorus" has a literal meaning, she tried to record a song of her with Justin Bieber but he sang so badly she just took him out, you can find it in youtube easily. Fantastic reactions tho 👌
Yes I heard this too and I heard the chorus he sang for her. But it could be a double meaning too because obviously he didn’t put the effort during that studio recording