Amnon is a lovely name. I wouldn’t let the actions of one person tarnish it too much. Many people, from A to Z, have also done horrible things. Have a talk with your own Amnon about the true meaning of the name, why the Biblical one was wrong for what he did, and how an individual’s name is unique and special to him or her. Let your Amnon know that he (or she) was *not* named after Tamar’s half-brother, but rather in the hope that your child will be a good and faithful person. Just my two cents. :-)
I wouldn't name my child this. Amnon lured his sister into his home under the guise of being sick. He then raped her. People familiar with the tale will recognize the name. I wouldn't want to start my child off with a negative connotation. It's a shame.