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)
By preventing forced sacrifices an answer that dodges hexproof and indestructible she keeps your board intact while applying steady flying pressure
You can use it to recycle your used Ritual Monster or Spell from GY or return them from hand to your deck where they shine best when searching for a Voiceless Voice Second, you can target cards your opponent controls, up to the number of LIGHT Ritual Monsters (Warrior or Dragon) you control
Diseased Vermin might make for an alternate rat if you must have a rat, especially if you can give it evasion somehow, and it works with lifelink
This fun thang is pretty strong from several directions
The same method can be used to modify the deck once it's built