Our Plague Dragon friend is one of the many candidates while the slime guy is a slime who ate a candidate gaining their memories/skills. I'm at the latest translated chapter (230) in the novel, and what a candidate is still hasn't been answered though.
A spoiler candidate is a non-challenger that doesn't have a likelyhood of beating the incumbent, but nevertheless may jeopardize the chances of the incumbent winning. In this sense, VoG is saying "He's not a challenge, but he'll be a threat to the goal I set out for you nonetheless if you don't do anything about him."
Unfortunately, the slime is not the largest asshole in this series, that honor would belong to the next antagonist, Irusha, the hero.
He has a inferiority complex, and frames the captain of the guard of a minor city for murdering both the captain's wife and brother-in-law. It is implied that the hero has killed dozens more, and he actively participates in slavery. When he was cornered in a fight, he released the magical equivalent of a dirty bomb in order to escape, and when he got cornered again, he held a toddler, who still believed him to be the hero, captive before attacking a crowd with a cursed sword to regen hp.
And that's not even the worst thing he did. It will take two years to get there, but when this upcoming arc comes to a resolution, it is both satisfactory and bittersweet.