I decided to write a bit more about what I know about historical berserkers, or at least how they were seen by the people who wrote the sagas. It got a bit longer so I rather post it here, than filling the patch discussion with it:
The main problem with the beserker is that the meaning of the word shifted so much. For all I know the name was used first for a group of southern germanic Chatti warriors meeting the romans who actually wore animal skins and fought in a frenzy.
It seems that this kind of berserkers were still around in the later viking age, although I can not remember animal skins being used. Instead they were "ordinary" people who just were so aggressive that in some saga they are required to leave the ships once in a while to wrestle with trees to calm down. It was also said that iron couldn't hurt a true berserker.
I have also read about a second type of berserker being body guards for renowned leaders. Sometimes it is mentioned that these wore animal masks, but I only read about this rarely and don't know much about this type. Still it is probably obvious that for this task it would be a bad choice to ask somebody who
wrestles with trees if he would like to be around somebody whom you wan't to stay alive.
Then I also remember a last type of berserker that is seen very negatively, appearing in the late viking age. Appearantly the custom of these times was that free men could be demanded a duel from, and if they declined they were seen as honourless. The duel was not for live and death, but with the weapons used it wasn't rare to die, and since the winner was seen as more manly he inherited all of the losers posessions. And so some people who were reckless and had nothing to lose just demanded duels from everybody they hoped to win against, and these people were usually despised. They didn't actually have to go into a battle frenzy, their most striking feature is rather how little they cared for their fellow people, and they just took what they wanted from them, and if it was reclaimed a duel followed. And since many people didn't just want to die for a necklace or whatever they rather just gave it (although this was unmanly). Now of course one can hardly put all those people mentioned in the sagas into one of these classifications, but this last type of "bully-berserker" seems to be very usual for the later Viking age, and often there are many of these three traits in those berserkers mentioned.
Now so I can keep up with Ulfvaldr, and you see I didn't make it all up I can quote a bit from Egils Saga from the
Icelandic Saga Database. In the first bit it is mentioned how Thorolf, Egils brother, is overcome by bloodlust when fighting against the Scots as a mercenary of the English king [chapter 53]:
Then Thorolf became so furious that he cast his shield on his back, and, grasping his halberd with both hands, bounded forward dealing cut and thrust on either side. Men sprang away from him both ways, but he slew many. Thus he cleared the way forward to earl Hring's standard, and then nothing could stop him. He slew the man who bore the earl's standard, and cut down the standard-pole. After that he lunged with his halberd at the earl's breast, driving it right through mail-coat and body, so that it came out at the shoulders; and he lifted him up on the halberd over his head, and planted the butt-end in the ground. There on the weapon the earl breathed out his life in sight of all, both friends and foes. Then Thorolf drew his sword and dealt blows on either side, his men also charging. Many Britons and Scots fell, but some turned and fled.
Now it could be argued how berserk-y this deed was, but Thorolf is not mentioned being a berserker, so this has to be seen as normal behaviour.
On the next day of the same battle though, Thorolf is killed, and his brother Egil becomes equally furious about this, so he follows his example [chapter 54]:
And now the armies closed, and soon the battle waxed fierce. Thorolf pressed eagerly forward, causing his standard to be borne onwards along the woodside; he thought to go so far forward as to turn upon the Scotch king's division behind their shields. His own men held their shields before them; they trusted to the wood which was on their right to cover that side. So far in advance went Thorolf that few of his men were before him. But just when he was least on his guard, out leapt from the wood earl Adils and his followers. They thrust at Thorolf at once with many halberds, and there by the wood he fell. But Thorfid, who bore the standard, drew back to where the men stood thicker. Adils now attacked them, and a fierce contest was there. The Scots shouted a shout of victory, as having slain the enemy's chieftain.
This shout when Egil heard, and saw Thorolf's standard going back, he felt sure that Thorolf himself would not be with it. So he bounded thither over the space between the two divisions. Full soon learnt he the tidings of what was done, when he came to his men. Then did he keenly spur them on to the charge, himself foremost in the van. He had in his hand his sword Adder. Forward Egil pressed, and hewed on either hand of him, felling many men. Thorfid bore the standard close after him, behind the standard followed the rest. Right sharp was the conflict there. Egil went forward till he met earl Adils. Few blows did they exchange ere earl Adils fell, and many men around him. But after the earl's death his followers fled. Egil and his force pursued, and slew all whom they overtook; no need there to beg quarter. Nor stood those Scotch earls long, when they saw the others their fellows fly; but at once they took to their heels.
But neither of both are seen as berserkers at this time. Indeed the only person in this saga who is consiered a berserker is a man named Ljot, who is the type of "bully-berserker" I described earlier. But he makes the mistake of demanding a relative of Egil, Fridgeir, for a duel and Egil fights in his place [chapter 67]:
These were the laws of wager of battle in those times, that when one man challenged another on any claim, and the challenger gained the victory, then he should have as prize of victory that which he had claimed in his challenge. But if he were vanquished, then should he ransom himself for such price as should be fixed. But if he were slain on the field, then had he forfeited all his possessions, and he who slew him in the combat should take his inheritance. This was also law, that if a foreigner died who had no heir in the land, then that inheritance fell to the king's treasury.
They now start, and soon come to the island. There was a fair plain near the sea, which was to be the place of combat. The ground was marked out by stones lying round in a ring. Soon came thither Ljot and his party. Then he made him ready for the combat. He had shield and sword. Ljot was a man of vast size and strong. And as he came forward on the field to the ground of combat, a fit of Berserk fury seized him; he began to bellow hideously, and bit his shield. Fridgeir was not a tall man; he was slenderly built, comely in face, not strong. He had not been used to combats. But when Egil saw Ljot, then he sang a stave:
The fight is a bit longer, but eventually Egil just hacks of one of Ljots legs which ends the berserkers life, and Egil has fun making funny little poems about it.