What's the reward for killing Alira, Oak, and Kraityn?
Solution 1:
If you kill all three of them, you will be rewarded with two passive skill points.
Solution 2:
Regardless of difficulty, if you kill all three bandits, you will receive one passive point. The reward for siding with a Bandit differs based on the difficulty. Who you side with can change in each difficulty. If you side with Oak in normal you are not limited to only choosing Oak again in cruel or merciless.
Normal Difficulty
- Alira - Permanent increase of 60 Mana
- Oak - Permanent increase of 40 Health
- Kraityn - Permanent increase of 10% to all elemental resistances
Cruel Difficulty
- Alira - Permanent increase of 5% cast speed
- Oak - Permanent increase of 16% to physical damage
- Kraityn - Permanent increase of 8% to attack speed
Merciless Difficulty
- Alira - Permanent increase of maximum power charge by one
- Oak - Permanent increase of maximum endurance charge by one
- Kraityn - Permanent increase of maximum frenzy charge by one
(Source)
Solution 3:
Although @Domocus was correct at the time of writing, as of the 3.0 expansion these rewards have changed. There is now only a single playthrough (rather than three separate playthroughs at different difficulties), which results in each character completing the bandit quest only once.
The rewards for doing so are:
- Help Alira: 5 mana regen/sec, +20% global crit multi, +15% to all elemental resistances.
- Help Kraityn: 6% increased attack and cast speed, 3% chance to dodge attacks, 6% increased movement speed.
- Help Oak: 1% of life regenerated/sec, 2% phys damage reduction, 20% increased phys damage.
- Kill all: 2 skill points.
The details of this quest can also be viewed on the game's wiki.