1. "seemed" -- but not actually?
2. "try" -- but not succeed?
3. "hammer" -- the very POINT of all this is, this issue can't BE hammered and one shouldn't try to treat is as such a simple yes-or-no problem.
There is a saying about that -- when all you have in your problem-solving toolbox is a hammer, every problem starts to look like a nail.
About "violence" being a natural human tendency or trait -- violence with a specific, premeditated object and no immediate survival purpose, no. That I will agree is probably more open to choice than people usually admit or make the effort to control. But "aggression", as distinct from "violence", is a trait present in many, many species, either as a defense or as a way of getting something needed or wanted for more or less survival purposes, or for bettering chances at "success" (meaning dominance, which is itself a survival purpose really). Aggression is the motivating force; violence is the quality of the act that has been motivated and put into action. Petty distinction? Not really -- the space between the two is, as William Blake (I think) may have said, "the moment Satan cannot touch." (No, I don't believe in Satan; it's a figure of speech. I think humans are plenty bad at their worst to explain all the ills we perpetrate or suffer, and want to blame on the existence of "evil".) The distinction between aggressive impulse and violent act is the site of choice; and we do have the cortical development to delay the act by reason. That is the relatively new, orbital-frontal cortex sitting very near all the limbic structures Jillian is mentioning, and it has evolved to put the brakes on the more shoot-from-the-hip impulses that usually result in flight or fight behaviour. The brain has evolved a fiendishly complicated architecture, with new structures on top of old, and sometimes the older and newer functions are at odds with each other. So it is possible for the higher human functions to override older survival impulses. The example I sometimes give to students is Pickett's charge -- how can those good men on those poor horses have run straight into the guns like that, knowing they could not possibly survive it? (This is someting else humans can do with all that reasoning power -- sublimate aggression that is not likely to succeed into an act of sacrifice.) Stressed sufficiently, though, especially as individuals without a group purpose to gather and drive us, most of us will discover that the older portions will take control.
However, human reasoning can be used to make the choice for aggression too -- it is not just some superior state of mind resulting in aggression-proofing. If reason itself is threatening, it can provoke aggression in the one being reasoned with. I give you Charles Schultz:
Lucy: I'll get you, Charlie Brown! I'll get you! I'll knock your block off! I'll....
Charlie Brown: Wait a minute! Hold everything! We can't carry on like this! We have no right to act this way. The world is filled with problems. . . people hurting other people . . . people not understanding other people. Now, if we, as children, can't solve what are relatively minor problems, how can we ever expect to... (POW).
Lucy: I had to hit him quick. He was beginning to make sense!