Just a few episodes later, they're going to bring back Tasha Yar only to give her a fate worse than death.
Tasha had to submit to repeated sexual assaults in order to secure the lives of the Enterprise Cee's surviving crew, then subsequently give birth to the rapist's child. Her last thought's were that her own daughter betrayed her.
Armus at least killed her quickly.
Riker has absolutely no grounds for interfering with the legal process of a civilisation that is not a member of the UFP.
What "legal process?" He stopped a murder.