I purchased a horse several years ago I believed was "round penned to death". The trainer I purchased him from is a fantastic trainer (you never hear a bad word about the guy from anyone), but this particular horse should have gotten out of that round pen and into the wide open space. He was SO NERVOUS. It took a month to get him to relax and walk on a loose rein out in the open. He didn't do anything aggressive at all, but was just super nervous about constantly paying attention to you. I think, in part, of being over-worked in the round pen by someone who does it right. Point being, even if you are working your horse correctly in the round pen, it should still be used moderately, as round pen sessions can be intense when you are contantly expecting that horse to do exactly as you say based on your body language.
I agree 100% with Cherie.
Any horse that shows aggressive behavior has had that problem created by the handler, 99% of the time. I say "99%" because there are always those whacko exception horses who are agressive/nuts on their own, although very few and far between.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.