Based on the meaning of "guard" or "watchman":
* Attacker: This is the opposite in terms of action, as an attacker is someone who tries to harm, while a sentry protects.
* Target: This is the opposite in terms of role, as a sentry is the one watching out, while a target is the one being watched.
* Sleeper: This suggests someone who is not alert or vigilant, the opposite of a sentry's duty.
Based on the meaning of "position":
* Interior: If a sentry is stationed on the exterior, the opposite would be an interior position.
* Rear: If a sentry is stationed at the front, the opposite would be a position in the rear.
Based on the meaning of "being vigilant":
* Careless: This suggests someone who is not attentive or cautious, the opposite of a sentry's vigilance.
* Neglectful: This implies someone who doesn't fulfill their responsibilities, which is the opposite of a sentry's duty.
The best antonym will depend on the specific situation and how you are using the word "sentry."