Is your rat around 6 months to a year old? If he is, he can be at an age where testosterone could be overwhelming him-- it can cause him to be aggressive towards other rats and, if you even smell the least bit like another male rat, it can make you a target for his aggression.

Here's a good article on this:
<http://www.rmca.org/Articles/aggression.htm>.
The author recommends neutering (your rattie really couldn't help his bad behavior) rather than having a rattie put down. Here's a bit on that procedure:
<http://www.rmca.org/Articles/qna2_97.htm>.

This article
<http://www.icubed.com/users/mfichten/rats/biter.html>
would be helpful if you'd rather try "rehabilitation" rather than neutering.