If something is not illegal but clearly bad, it's a lesson that we should change the law. It absolutely should not be legal to lob a completely baseless murder accusation at someone with no evidence.
The problem is that "changing the law" usually changes the legality of more than just the specific situation at hand. So a law banning "my tarot cards tell me person X is a murderer" may also ban "I've carefully reviewed all the evidence, and I think person X is a murderer" where this is a reasonable accusation. And we can go from murder to "rape" or "abuse" etc. You need to be very careful with this kind of stuff; i…
Countries that extend defamation to opinions tend to apply a reasonable person or similar standard to the opinions.