The Remaining Saviors Kill Him
While there seems to be only a few Saviors left that were part of Negan's killing gang, it doesn't mean there aren't any left. The Saviors are feeling pretty hunted nowadays and we all know what happens when people feel cornered in the apocalypse. Right now Rick seems to be the only one truly keeping the peace between all the communities, but he's also the one giving the firm decree to not arm the remaining Saviors. This definitely rubs a few of them the wrong way, and who knows how they might decide to retaliate in the future.
Image Source: AMC