yes it is illegal, it's a forced torture, it will not stop your child from doing bad things but it will damage his spirit forever, your kid will have personality problems, be depressive with negative attitude...

Washing a child's mouth out with soap is child abuse!
take responsibility for your child and act like a responsible adult! Screaming, yelling, beating and torturing your child isn't going to help it's behavior!

my nan done it to me when i was little and it was so horrible the tiniest bit will make you either throw up or want to throw up and no matter what i will never forget that day and i don't know why you'd want to do this as it is not teaching your child anything and your child will probably remember this and may hold it against you.

using force could never be a sollution for such problems especially with kids it will threaten their personalities and then they feel fear from everything so they will have weak decisions