It depends on your definition of "Viking" I suppose as not every Norseman was a Víkingr (or "raider"). There was obviously the Vikingr raid at Lindisfarne & the occasional exploration, the Norsemen were quite progressive. I'll ping /u/Eyestache & /u/textandtrowel who should be able to answer your question in more depth.
They weren't, really. No more violent than any other raider, outlaw, or soldier. When a man went on víking, he generally went with the express purpose of taking plunder to increase his wealth and social standing. Occasionally, that did involve killing people who would otherwise rather that their goods weren't taken, but that's the reality of living that sort of lifestyle.
When you look at the depredations of other contemporary outlaws or military forces, you'll see a very similar pattern emerge when raiding: Show up unannounced, defeat or deter any resistance, plunder everything you can - goods, animals, slaves, etc., and then take off as quick as possible to avoid retaliation.
I'm curious, though, is there a particular reason that you think that víkingar were particularly violent?
I have heard two interesting ideas recently when looking into 'Viking' history. Firstly would agree with subs-man in that Viking is a pretty general term in that the people from that area went all over the bloody place in that that era. In general invading and plundering but in some cases, such as the Normans who (although initially being pretty forceful) adapted and took on the Frankish culture so quickly that they were almost unrecognisable after a relatively short time); and as the Rus who as far as I ma aware and according to Islamic sources from that time were mainly involved in trading. Like Eyestache point as well - they were pretty violent times and in England people often forget that the Saxons who the Vikings were invading did the same thing (although potentially more of a migration an intergration depending on your viewpoint; but not if you like the King Arthur legends.....the real ones, not the later French elaborations). The Angles, Saxons and Jutes were from similar initial areas to the Viking invaders to Britain and war fare and life in general was probably pretty brutal all round. There are plenty of findings to suggest it went both ways - although most sources from then are written from the 'defenders' pov so would paint out Vikings as horrible anyway.
Anyway - one idea I heard was in the book by John Clements in the book 'a brief history of the Vikings' which isn't all that brief but is excellent and really covers the breadth of the Viking excursions of that time (not just the English invasions). His POV is that there was a fairly functional and civilised (for the time) society in these areas at the time and that anyone leaving to raid, plunder and make their fortune elsewhere was potentially either pretty violent and unsavoury or desperate, or in the search of glory. So this kind of fits with the general idea of what they were like and why.
The other idea, and apologies as I forget where I heard it, was that The Frankish incursions under Charlemagne who went around slaughtering Pagans in the area in the name of Christianity, may have stirred up a hornet's nest as it were, and some of the raids were motivated by political/theoretical reasons (or at least under that guise) - my Gods (are) better then yours type affair! Or even a case of revenge.
Anyway, love the debate. Could go on and on! That's why we love the 'Dark ages.'