Can a convicted sex offender sue me for Slander

First, revealing the truth is not defamation.  I'm sure you've heard that "truth is an absolute defense."  Apart from defamation, it is possible that you violated some confidentiality statute by releasing this information (I don't know the particulars of Washington law) but not if the information was in the public domain.   Second, even if you had defamed the offender, the damages would be minimal.  Defamation damages are measured by the damage to the defamed person's reputation.  If a person't reputation is already terrible, it is unlikely that a false statement will materially worsen it.  Thus, if you claim that Mr. X murdered 10 people, when really he only murdered 8, it may technically be a tort, but Mr. X is unlikely to be able to recover anything.