Yes, I believe Rick Grimes was justified in brutally slaying the cannibal, Gareth, with a machete. The Walking Dead is a TV show, and Rick Grimes has the right to do whatever he chooses in the story line. If he wants to brutally slay Gareth with a machete, that is his choice.
One word - cannibal. The man was a cannibal, and we all know that Rick Grimes basically did nothing useful up until the time he killed the prison guys just in case they were going to be a threat. His choices are stupid and his word never carried out, so I think it's wonderful that he finally tried to protect his group by killing off a cannibal.
Yes, I think Rick Grimes was justified in brutally slaying the cannibal, Gareth, with a machete. He did what he needed to do to protect himself and those around him who needed protection from Gareth the cannibal. Gareth posed a huge danger, and Rick felt it was only a matter of time before Gareth harmed him.
He was justified in slaying Gareth the cannibal. This is because cannibals are terrible individuals. They eat their own. Anyone who does this is full of bad news, and only views others as potential lunches. Rick Grimes did the right thing on The Walking Dead. I doubt most people would disagree.
The lack of imminent danger prevents there from being a generally acceptable moral or ethical outlet that would justify the way this went down. Not that it matters because it's a TV show and worrying about real people requires more brain cells. So in TV land of course he was justified.