It is hard to define the word, but it is basically used to describe someone so disgraceful that they are seen as the lowest form of life. “Worthlessness”, “waste of skin”, “dirt”. “Nothing”. Far worse than most other insults, where the victim is often just referred to as genetalia.
The fact that “scum” is a non-swear, and seen as perfectly acceptable English, makes it even more demeaning and offensive. If you are condemned as a “cunt” or a “fucktard”, then you are simply being attacked with cliché profanity, used spontaneously and with no particular venom or thought. “Scum”, however, is usually only used in exclusive conditions where it is the only word for the job, and makes the victim feel like the lowest of the low.
You are scum.