Brutality grants you 50% chance that your combo which kills the target will be converted into Fatality (this is not exacly what tooltip says but it's what it really does)

Few facts about brutality:
- the proc is 50%, but from what I have seen in PVE if you do not fatality first mob, the proc won't go off on another. Same if you proc fatality, you will chain fatality next mobs.
- fatality animation does damage to other mobs that are closeby (AOE)
- during fatality you cannot be affected by melee or spell, but you benefit from healing already cast on you (spells, pots)
- fatality gives +20% dmg buff and regens stamina very fast
- only combos proc fatality
- only "killing" combos proc fatality - if you do less than enough damage to kill a mob, fatality won't proc.
- Antrophy for some reason is not geat for procing fatality (or perhaps I have been very unlucky with it)

PVE brutality:
Brutality affecs you and your team so you will benefit from it in PVE aswell. I have had a blast in PVE when fighting 3-4 mobs at once which is not easy thing for lotus assassin (without droping all stuff at once, and then waiting for a cooldown).

Timing
The brutality should be activated on your LAST combo step and only if it is "killing combo". Any other usage like droping it at the begining is wasteful. When I am faceing group of mobs that I will use brutality on I open with GC to:

1. take out chunk of health
2. debuff
3. FORCE ALL SHIELD to the LEFT (helps SSIX to be killing combo)

After that I proceed with SSIX and buff it with butality.

Targeting:
If all mobs have equal HP - always target mob in the middle to do most AOE damage
If one mob has lowest HP - target him for more reliable fatality proc

Chain
The best brutality usage is "chaing fatality" - since the brutality buff last 15s you can get 2-3 procs of it and jump from the mob to mob. It is really spectacular view to see assassin grab his enemies one by one and lay the smackdown. To get chain use GC after first fatality if mobs have more HP (meaning they will survive the opener and die from combo finisher) or SLAM if they are almost dead.

use SLAM
Slam is your best friend when it comes to procing fatality - it does instant damage so it won't kill the mob with combo step hit and it proces OFFHAND nicely to help you do that extra damage for fatality proc. Use only on REALLY damaged opponents.

The most important TIP:
If you are lotus assassin chances are you are specced into Sense Weaknes which grants 25% chance to crit on your combo opener. I know all of us <3 big critz, but having your GC opener crit and kill mob is not something you want when you have to fight the other two without fatality proc. So, when your target is low on health (but not low enough to kill him with SLAM) turn from the mob so your GC opener hits air, and then turn back to land the GC's combo finisher and enjoy the fatality. If you use mouse look turning you can do this very fast and fatality proc is WORTH the hussle.

PVP BRUTALITY
all things from PVE apply here except it is much harder to use it well in pvp. Since fatality buffs everyone in the team, it doesn't have to be you who fatalities someone. The fatality buff can be real "battle turner". Brutality is great BUT - 3 points is a lot when it comes to PVP specc which has to include antiCC stuff. Is brutality really worth the 3 points in PVP build? That is up for a debate.