I'm gonna agree with passionaro and SK Ren here.
However, even with SK Ren's idea it doesn't deter people from being a pansy quite enough.
Simple solution: Berserk packs give 2 (or even 3) extra levels instead of one. Berserk packs don't last as long. (Perhaps 30-40 seconds)
Any form of 100% or 200% healing (aside from picking up a berserk pack, of course) reduces your berserk level back down to 0.
This solution means that berserk packs will give you a quick fix of berserk (2-3 levels, but it only lasts for 30-40 seconds), good for starting off your killing spree but a poor way of keeping it going. The healing clause just means that unless you have MVp (which would imply you've killed a lot of enemies anyway), it's difficult to keep berserking after 25 seconds, unless you have a lot of small medpacks.
Complex solution: A variable keeps track of how long it's been since you last did a melee attack. The higher it is, the faster your berserk time drains away. This variable is reset back to 0 whenever you go down stairs.
This solution stops people from not killing anything (and gives them a decent way to de-serk without being forced to tab back down to 0) but may make AoB too complex, as there's already a few variables for the berserker to keep track of.
Now that I've typed them out, I think the first solution is better. Or both could be implemented at once.
The new system with passionaro's and SK Ren's ideas added:
1:Angered: +1 armour, x1.5 melee damage, +2 melee accuracy
2:Enraged: +2 armour, x2 melee damage, +4 melee accuracy, 0.8 movespeed, 0.8 attackspeed
3:Furious: +4 armour, x3 melee damage, autohit melee, 0.7 movespeed, 0.7 attackspeed, takes one damage per every turn in which is not in melee
4:Frenzied: +6 armour, x4 melee damage, autohit melee, 0.6 movespeed, 0.6 attackspeed, takes one damage per every turn in which no foe is slain
5:BERSERK!: +12 armour, x5 melee damage, autohit melee, 0.5 movespeed, 0.5 attackspeed, takes 4% of maxHP damage per second.
10:Crazed!: Same as BERSERK!, but you take 7% of maxHP damage per second.
15:Insane!: Same as BERSERK!, but you take 10% of maxHP damage per second.
EDIT: I was going to change this a little bit, but then gargulec posted a simpler idea so I've edited to his.