They don't necessarily tear anyone up unless you have too few hens per roo (optimum is around 8-10 hens/1 roo), but even then roos often choose a few favorites that they'll breed more often than the rest. Perfectly normal. Sometimes, those favorites (or all of them if you don't have enough hens) can get broken and missing feathers on the back of their heads, backs, and wings. It depends on the roo, though. You may also end up with a person-hating feathered fury, too.
If you're interested in getting a roo, though, I'd recommend finding one that's at least one year old and making sure you discuss his disposition with the current owner thoroughly before making an commitments. Older roos tend to be calmer than their horomonally erratic youths and not so driven to mate everything in sight.