If you're antagonistic, you're hard to get along with. Harmony is not your friend — you prefer hostility and struggle.
If you're antagonistic to your little brother, you're always picking on him and giving him a hard time — causing him agony — a word that's buried in antagonistic. Even drugs or chemicals can be antagonistic: if you take a sleeping pill and then follow it with a few cups of espresso, the interaction is likely to be antagonistic. The sleeping pill and the caffeine struggle against each other and you're not likely to get much sleep.