Why are mixers legal? The only purpose I find on them is to money launder. They are literally useless if you don't have anything to hide.
Probably because governments still don't care that much to look into regulating Bitcoin.
Recently a man was deceived by someone who had change the electrum source code on an old version of it. He lost 1400BTC(!)
This is wrong, Electrum's source code wasn't compromised, what are you referring to is a bug/oversight that allowed Electrum servers to send messages to clients that looked like wallet notifications.
If he send 1BTC to 1400 owned addresses, though, no one can prove that the person that owns those 1400 addresses is the hacker, right? They can only ask them where did you get that one bitcoin.
It's actually fairly obvious that all those addresses belong to a hacker or otherwise linked to him, because such strange transactions don't happen under normal circumstances.