Should it be relevant though? It seems to me like criminalization of thoughts. Even if they externalized into a diary.

If you were caught with notebooks detailing your plans to kill a list of people, showing that you've meticulously tracked their movements and listing locations for dumping the bodies that would be extremely relevant. I don't see how it'd be a good idea to exclude that kind of evidence.

If you write in your diary "I'm gonna kill her" and then she gets killed it's relevant

Depends, if you wrote a detailed confession with material non public facts, a jury can hear it and weigh the evidence.