100 % activating but 100 % of what?
On criticals or on the first shot?
Any Attack.
It is like a permanent buff?
No, it doesn’t proc on any attack. Just tested. Rather on critical hit.
This.
The description is a bit ambiguous, but the Deadly Aim has a 100% chance of procing on crit.
Its activation on crit, the tooltip is wrong. I just found out that it only triggers on crit, stays on for 5 secods after that, and can be re-triggered after the cooldown of 5 seconds has run out. So if you crit while its active, it wont stay for another 5 seconds.
Of course, that would be entirely imbalanced. Its brutal enough as it is, given that you can land any crits (say on bosses). Invest in cunning, or you wont have fun with this one 
Thanx i wasn’t sure , but the description is indeed kinda ambivalent.
I wouldn’t rush for that. There are very few builds that can use cunning investment reliably. You get enough OA from gear/devotions
I havent played the game much past the first playthrough on Veteran (then went on to try out other builds until i got bored), but the builds I had most fun with where cunning heavy. I am now trying out a dual pistol purifier (demo/inquisitor) with a 1/2/2 split for attributes. Sounds glass cannon, plays like one so far. Loads of fun though
Deadly Aim isnt a game winner atm (few crits at the beginning… I know you can easily breach 20% crit rate, but that is until midgame. Now late early game, because they added chapters G). Its nice when it procs - face stomping time - but it doesnt often, and the enemies rarely stay for a prolonged beating. I would have prefered if it just went off on any attack for one attack, then triggered cooldown (that went down to like 3 seconds if you spent in it). But oh well, its 5 seconds of glory after a crit atm.
Can I activate deadly aim using word of pain?
yes you can.
Deadly aim can not activate from pets or pseudo-pets (like the runes, the inquisitor seal, wind devil, storm totem, wendigo totem, etc)
If I remember correctly, I believe my infiltrator’s deadly aim would trigger with the Night’s Chill aura too. Running by dummies in town would kick it on all the time.
Yes. Any crit. Including damaging auras like Censure or Whirling Blades.
Excluding pets. I’m not sure if pseudo pets counts but I think they do. Easy to test on the dummy.
I have tested with pseudo pets, do not work
at least not for these:
Wendigo totem
Storm totem
Wind devil
Rune of Haggarad
Rune of Kalastor
Inquisitor seal
Any crit will activate Deadly Aim
its more complicated than that