Well, or say it's not murder but the premeditation of one. There's no physical evidence, only maybe a chat-log. And in that case, although it's not really a crime yet, the police could hire an investigator, etc.
I guess you are within your rights to give evidence anonymously. In that case questions cannot be asked as to where the information came from, however it may not be enought to warrant an arrest, but would probably be enough to go on to obtain the information in a legal way, i.e. search warrant so that the computer's files could be extracted in an official manner and given as evidence. I doubt a chat log by itself would ever be enough as I could spoof one now but the problem is that there isn't any evidence of its originating source. The evidence could then however lead to the confiscation of the computer and maybe even a subpoena on the internet activity logs ISPs are meant to keep by law for a limited period of time. Thats when they play the "my router isn't secured, anyone could have connected to it by spoofing my mac address" card lol.
I didn't feel like reading everytthing everyone posted but Phred is right.
a) if you hacked in, you can plant evidence
b) the person b will be investigated
c) you do get in trouble because regardless hacking is illegal no matter what