I think you misinterpret the use of greatest in this title, greatest under this context simply means the biggest, so linking it to revenge, it simply means the most brutal and vicious acts of revenge. So it doesn’t mean acts of greatness. Greatest basically has two different meanings, so whereas it can mean something that is great, it can also mean the biggest, the most substantial. For example, that is why Great Britain is called Great Britain, because the island is the biggest island of the group of islands.

Hope this clarification makes sense! And thanks for reading! :-)