1) What he did was illegal. Those aid programs and the funding they were to receive was written into law by congress - elected by the people - with overwhelming bipartisan support

2) No of course we are not necessarily morally obligated to donate aid in perpetuity. We can decide to stop if we want to. But it was not Musk's or even Trump's decision to make. But since they assumed that mantle of illegitimate authority, they assumed the moral blame that comes with it.

(And yes, every Trump voter also shares significant moral blame for the deaths of all those people. Atrocity was a predictable and likely consequence of a Trump vote in 2024)