That M*therf***er. I know it's pointless to stress about it, as the author somehow decided to stir the plot into that direction, but...
he was glad she died, only married her for power and lied to her all the time before. He managed to lie to her all the time! He managed to pull that off! I ain't the only one, who sees how dangerous such a thing is, right? Girl, what more reason do you need to conculde he is not trustworthy? And nether is her own judgment. What insurrences are there, that he doesn't pull the wool over her eyes again? Right now?
If somebody lied to me to that degree and I somehow noticed it (FtLoG, she had to die to see it!), I'd remove that person without another word to share. (Just because I know my own mind would try to compel myself into believing, that everything is a mistake and that person isn't really that evil. I wouldn't trust myself on this.) And what if there is someone even worse out there? Still doesn't make him trustworthy. You simply don't let sombody like that take charge of his own "investigations", let him realm free to spook real witnesses, destroy evidence and listen to his BS afterwards. Cut his neck and be done with it. Even in case he didn't kill her, he is still a fiend and shouldn't be allowed to outlive her for the sake of the kingdom's future.