In most cases, the word manners is plural. When one is speaking of manners they are referring to characteristics or a set of social customs of an individual, such as in your last example "His ill manners are harmful to his children." In this sentence, as with the word itself, it doesn't necessarily refer to an isolated action. It instead looks at a set of actions of the father collectively.
In fact, the word manners itself is often times synonymous (the same as) with the word "behavior."
Hope this helps you!
- For Teachers