Is it illegal to destroy us coins?

No. If you own the coin, it is your loss if it is destroyed. The idea that it is illegal to deface coins goes back (at least) to the Roman Empire. Money belonged to the Emporer and the people were only allowed to use it. It also had his picture on it and damaging his image would indicate your disrespect for his authority. There is, however, a temporary regulation from the US Treasury forbiding the export or melting of cents and nickels for their metal value because the current market price slightly exceeds the monetary value of these coins.