What does it mean? Terrorise ...explicable...

It makes his willingness to terrorize { to make people very afraid of losing their lives } explicable { to state the reasons or an explanation for doing in order to make something understandable }, if not defensible.

It is saying that it (the unstated background) gives the reader some insight as to why he was so cruel, even though that information won't make anybody think such cruelty is , even after the excuses are made, forgivable.

Joe(There's a lot packed into those two words)Nation

Note: The British terrorise, American terrorize..... wait a minute, that doesn't sound right.

It (whatever "it" is) makes it easy to understand why he (whoever "he" is) would be willing to terrorize people, even though this undertsanding does not excuse his behavior or make it defensible at all.

Got it.
Thank you both.