Just my two cents, he's a textbook narcissist. Would he accept responsibility, no. Is his ego so big he thinks he can actually start WWIII and nuclear armageddon, absolutely. Add a dose of kompromat et vois la, Elon Musk is now sympathetic to Russia.

So would elon accept responsibility for terrorists organizing on his site?

I mean, wasn't that the whole point of him purchasing Twitter, for domestic terrorist organizations' benefit?

Yes and no. This is specifically about one man who runs a company making a tactical military move at a precise time that was designed to hurt one country and help another. In other words, he just became an active combatant. The fact that he was willing to make such a move shows a clear lack of foresight.

What you suggested is what most ISPs do: they make general decisions about what clients they are willing to do business with, and after that it is left to each client. This is what Elon probably should have done, if he were thinking medium-term. But he didn't, so now all the major militaries around the world (including the U.S.) are rethinking how they want to deal with him and his company.