I'm going to break up with my boyfriend post major operation
That sounds like bad timing, why did you wait until now?
His singing voice is horrible and I'm going to kill myself if I have to hear him sing again.
Breaking up for someone because of their singing voice? Are there other issues involved here?
No. I. Just. Can't. Stand. His. Voice. Anymore.
Your allowed to not like his singing voice, but that's not really a reasonable reason to break up with someone, especially when they're going through a difficult time. Perhaps you should give it a bit and let him recover before you make any major decisions such as ending the relationship. Why not ask him to refrain from singing around you?