Base cost: -10 points; can be resisted.
Berserk is a mental, mundane disadvantage, described in Basic Set, p. 124. It has the special enhancement Battle Rage. It is referenced in GURPS Martial Arts, p. 179, on use of techniques by Berserk characters.
- On its pricing 
- On how the bad more than balances the good 
- On compatibility with Unfazeable 
- On its separation from Bad Temper in 4th Edition 
- If you must roll to stay conscious or alive while berserk, then you must roll – but you do so at +4. Once the altered state wears off, if you're in a state where you would risk unconsciousness or death, you must roll against HT for each effect, and without the +4. Snapping out of a berserk state is considered a shock sufficient to trigger further HT rolls. That's the top reason why Berserk is a disadvantage
- Ending your berserk state triggers one extra HT roll for consciousness at 0 or fewer HP, and one extra HT roll for survival at -HP or worse. Both are subject to the usual modifiers (e.g., -1 to consciousness rolls per full multiple of negative HP) and results (e.g., failure by 1-2 on a survival roll means a mortal wound). Neither enjoys the +4. These rolls represent additional chances to fail, over and above what you would face were you not a berserker.
- You don't keep a laundry list of every HT roll made at +4 and then reroll them without the +4 later; if you did, then Berserk would have a base value of -20 or -30 points, not -10 points.
- You definitely do make all of those survival rolls, one for each increment of -HP, at HT+4 when Berserk. When you snap out, you don't make them all again. You make just one roll at HT.
Angie the Barbarian has HT 12 and 14 HP. She goes berserk and starts whacking people. In the course of doing so, she takes four hits herself.
The first delivers 8 HP. Who cares . . . Angie can't be stunned, suffer shock, or slow down due to injuries. She's at 6 HP.
The next does 9 HP. That's 17 HP total injury, putting Angie at -3 HP. At every turn past this one, she must roll at HT+4, or 16 or less, to remain conscious. At those odds, she does, of course.
The third blow strikes the vitals, and inflicts 24 HP! That's 41 HP total injury, putting Angie at -27 HP. She hit one threshold at -14 HP (-HP), but not quite a second at -28 HP (-2×HP). She rolls at HT+4, or 16 or less, not to die. She doesn't die.
The fourth and final blow deals another 15 HP. That's 56 HP total injury, putting Angie at -42 HP. She hit thresholds at -28 HP (-2×HP) and -42 HP (-3×HP), and rolls twice more at 16 not to die. She doesn't.
Then Angie snaps out of berserk. Being at -HP or worse, she must immediately roll once vs. her unmodified HT of 12 to be alive. She does, luckily for her. Being below 0 HP and not dead, she must also roll once vs. HT to stay conscious – but at -3 for being so far in the hole. That's 9 or less, and she rolls a 10 and fails, passing out immediately.