I think that 3-prong test varies by jurisdiction/legal system. I was just going by the OED; libel: a published false statement that is damaging to a person's reputation; a written defamation.
It's never acceptable to "joke" about someone being a murderer unless you suspect them of being a murderer or everyone can see public records they have been convicted of same.
The first two was from my understanding of the libel laws in the US and the standard of proof that must be presented in order for someone to be 'liable' for libel.
The third bullet is my interpretation of US supreme court rulings on first amendment issues regarding satire of 'famous' people and libel.
I really cannot argue the moral issue of "joking" that someone is a murderer as this is not the appropriate venue for that and I do not have a strong enough of an opinion.
I would say that if you truly think there is a connection between his two comments and his ability to be trusted then the negative trust is appropriate. If you cannot make a strong logical connection between someone making this empty claim and their ability to be trusted then the negative trust is not appropriate. Remember that negative trust should be given if "you were scammed or if you strongly believe this person is a scammer"