Without a law that says USAID can be shut down or reorganized (or not funded) it seems that this is pretty much by definition entirely illegal regardless of one's opinions about USAID.
Pretty much everything they've been doing is illegal. The million-dollar question is whether they'll be held to account. Trump has a pretty good track record of facing little to no consequences for his crimes. The pessimist in me says this time will be no different. We'll see.
Be less obvious that they're out to destroy America. A hostile foreign agent would be too concerned with having their cover blown if they were that obvious.