In order for something to be considered libel, it needs to be

• untrue
• cause you actual harm
• said/written in a way that others would reasonably believe it to be true

I don't know you, and really don't know one way or another if you actually have killed someone before, but I think it is fair to say that you have not (at least for purposes of this discussion). You could probably argue that you have never been convicted of killing someone before nor have you been suspected of killing someone (both of which I assume to be true).

I am not sure about the other two issues. Do you seriously think that an empty accusation of being a murder without any evidence would override the trust that you have within the community? I personally would doubt so.

I do think that the OP's actions may warrant a temporary ban if he has shown a true pattern of this kind of behavior (making empty accusations without evidence - however once evidence is presented the accusation is no longer 'empty') as what he is doing could be considered to be trolling; however the pattern would most likely need to be a lot longer then just two posts