What do you think of violence in play?

I don't like violence. I don't really tolerate "violent play" because I DO NOT LIKE to have things thrown at me, be hit, be attacked in every way, I don't like it because it so quickly goes from play to aggressive violence to just violence (or "vileness"), and it seems to instantly become an unending trend when it is allowed, even just a few times. It feels like the kids cannot stop being "vile" when they have gotten away with it for even just a minute. But I can understand the thought behind the idea of allowing it.
Here is an excerpt from another blog (which actually has some really great ideas beyond this, which frequently seem brain-age appropriate for our (larger) little ones, and is linked to above)

"A word about aggressive play and toys: There is a misconception that allowing children to play with aggressive toys such as guns and knives will teach them to be aggressive. But child development research, and play therapy research in particular, shows children need to express their aggressive emotions in play in order to release them, rather than carrying them throughout the day. "

I don't know how to limit permissible -vs- "duh, no"... Kinda like I'm a little gun shy with letting kiddos try the nurse/doctor/"I'll show you mine if you show me yours" situations, and even playing Neopets because the site isn't just preschool-age games (though I've played NP for YEARS now and it's so many universes better than the Face*book games I'm letting the kids give it a try. Perfectly normal, perfectly age-appropriate (sometimes), perfectly paranoid because I know enough about my kids histories to know it's not a good idea.
So... How do you limit? How do you permit? How do you let them grow without getting shanked in the middle of the night?