"This is the kind of tired, predictable argument I expect from you."

 

"He had a mental illness, so we can ignore his argument."

 

This is where someone uses an insult in place of an argument. So they haven't done anything to show why your argument is invalid, they have just tried to gain the upper hand by attacking you directly. This is irrelevant, and is a sign that the opponent has run out of arguments and is resorting to cheap tactics. 

An insult can accompany an argument, such as, "The gospels are not eye witness accounts, you idiot!". That's not an ad hominem, as it is still making a valid point. But it could be considered rude and inappropriate to add the insult, which adds nothing of value to the statement.