As soon as Decimator Beetles ability resolves, state-based actions are checked, and the game sees that you have both counter types on your Servant, so one +1/+1 counter and one -1/-1 counter are removed at the same time (resulting in Servant dying)
Comments from Last Week Ill end by discussing a couple of the comments from my previous article about the Legacy format
While vigilance wasn't keyworded yet (it would happen in Champions of Kamigawa ), it was a white ability
So, you can have one copy of Thalia, Guardian of Thraben on the battlefield, but you cannot have two copies of Thalia, Guardian of Thraben
Magic was finally going to a world filled with its most epic monsters, so it was too good of an opportunity to miss partnering with the Toho Corporation and its iconic monsters such as Godzilla, Mothra, and King Ghidorah