Well, it's a crime and he has been impeached. But, that of course, does not mean he'll be convicted.
Why was the aid freeze illegal?
"Faithful execution of the law does not permit the President to substitute his own policy priorities for those that Congress has enacted into law," the decision by the GAO said.
The White House Office of Management and Budget (OMB) "withheld funds for a policy reason, which is not permitted under the Impoundment Control Act (ICA)", the ruling continued.
The 1974 Impoundment Control Act says it is illegal for the White House to withhold aid appropriated by Congress. It also says the White House must first alert Congress before it delays or blocked funds, which the Trump administration did not do.
https://www.bbc.com/news/world-europe-51139939