What is the difference between good satire and bad satire? Where is the line that some satire crosses that makes it just offensive instead of effective?
"Good" satire would be called Horatian and "bad" satire would be defined as Juvenalian
Horatian satire is light-hearted and funny (so, good). The appearance of Horatian satire would make the reader/listener/watcher laugh or smile since it is supposed to do that.
Here's an example of Horatian satire:
This is Horatian because of its funny and light-hearted attitude. The cartoon pokes fun at people who are obsessed with taking selfies and uses The Evil Queen from Snow White as a representative of those selfie-obsessed people.
Why isn't this Juvenalian? Well, Juvenalian satire is bitter and angry attacking. It picks at issues that are controversial (and sometimes not) and insult different cultures, races, disasters, issues and more.
Here's an example of Juvenalian satire:
This image is Juvenalian satire since it attacks the creation of the Disney American History Theme Park by mocking a photo from the Vietnam War.
The author picks a gruesome picture (I won't include it but it is a famous picture that depicts a girl running away from the smoke behind her) and mocks it, by drawing Goofy into it, indicating the infiltration and incorrect way Disney is creating the Theme Park.
The girl in the original picture is on Goofy's right side.
If you want more examples of Juvenalian and Horatian satire, follow this link to an answer that I've written addressing both styles of satire.
When satire become's simply offensive, and not effective, it isn't Juvenalian satire anymore. Remember, Juvenalian satire is bitter and angry attacking, but it also gets a point across, no matter how bitter or how angry the attacking is. If there isn't a main idea and it's just angry attacking, it's not satire.