Just like any sub-culture, there are going to be total fools that miss the entire point of what the sub-culture is about. These people have no heart, conviction, or honor (yet sing along to the atrocious amount of SxE hardcore songs about those subjects), and are only trying to prove how "cool" and "hip" they are. You should discount these people completely (or take the high road and try to educate them, which is VERY preferrable, but harder), whether you are Straight Edge or not. They will not stick with it.
There have been reports of "straight edge gangs" such as Monster Crew in Salt Lake City roaming about picking on people, but this is far from the norm. Monster Crew was responsible for pipe bombings of McDonalds and other places in Salt Lake, as well as a violent attack where a large X was cut into a guy's back with a beer bottle. However, these incidents are about the only cases of anything like this happening, and 99% of what you hear is fabricated rumor. So don't trust it. Like I said hatecore is pretty stupid but I have a few friends in Courage Crew and I give them respect as much as I disagree with their views.