There's a difference between an alleged "ignorance of the laws" and playing within the spirit of the game. True, a batsman leaving early might be taking an unfair advantage but, in most cases, they are anticipating the bowler's delivery. I believe a single warning is the best option and, if the unfair advantage continues to be taken, then a Mankad is warranted. As a bowler and a Test player, Broad would be aware of the laws of the game and part of his Tweet is "Hasn’t it always been a legitimate dismissal & whether it is unfair is subjective?" How can this comment make him an "idiot" or be taken as "ignorance"?
I have played almost 300 games and umpired over 150 and share Broad's views on a "Mankad". It's a legitimate dismissal but, IMO, unfair to execute without a prior warning.
I find it ridiculous that you cast such judgement on a man based on a Tweet which, when read carefully, indicates he was aware that it has always been a legitimate dismissal.