"Beaten on" vs. "beaten":
"Beaten" suggests a completeness, a thoroughness. It's equivalent to saying "the meal was eaten" - it suggests that people ate until there was no meal left. (When someone has been beaten thoroughly, we say they've been "beaten up".)
"Beaten on" is an idiom. It suggests a partial action - "I beat on him until someone stopped me," or "I beat on him until I felt better," etc. It doesn't tell you whether the beating was thorough enough to make him beaten up.
"Beat on" also suggests "commit violence again" more than "beat" does. If I say "I beat him," you might assume I won a game of chess against him or something. If I say "I beat him", you're somewhat more likely to assume I committed violence against him.
Passive Voice Vs. Active Voice
There's a difference in emphasis between passive and active. Passive lets you erase the subject/actor/doer.
You do this because:
- The subject isn't important. "I was fired today." I could say "Bob in the HR department fired me," but really, I don't care who fired me. I just care that I got fired.
- You want to hide or de-emphasize the subject. "Jimmy and I were playing catch in the living room, and the lamp got broken." vs. "Jimmy and I were playing catch in the living room, and we broke the lamp." The former sounds better than the latter. The latter emphasizes that we did it.