The things that come to mind at work:
The Martian Madness event can’t be concurrent to the cultists and Tablet being on the map right now, so my friend and I went to kill the cultists but lose intentionally to the Lunatic (we don’t want to start The End without our third member present) ago that we could start the Martian event.
While dodging about for several minutes to get to know the Lunatic mechanics, I really felt like his sound bank wasn’t necessarily evil-sounding…. It actually sounded…kind of sad… like pain and profound regret.
The cultists also don’t attack you unless you attack first.
So, headcanon time: the cultists aren’t there to summon the Moon Lord. They’re trying to hold it back. When you kill them, their ward weakens and the high cultist needs to step in to intervene, but it’s too late. In an effort to stop the inevitable, he absorbs the power of the tablet in hopes of keeping it in check, but it’s too strong.
So he goes insane and attacks you out of a combination of fury and madness, and killing him releases the power he tried to seal.