When Decimator Beetle enters the battlefield, put a -1/-1 counter on target creature you control.
Whenever Decimator Beetle attacks, remove a -1/-1 counter from target creature you control and put a -1/-1 counter on up to one target creature defending player controls.
Decimator Beetle’s second ability can target any creature you control, not just one with a -1/-1 counter on it. It’ll put a -1/-1 counter on the opponent’s creature even if there isn’t one to remove from your creature.
If one target of Decimator Beetle’s second ability becomes illegal, the other will still be affected.