His ult executes me for 331 true damage, 40-100 more damage than a 5-stack rank 1 Darius ult (if he has 0-50 bonus AD), and more than 33% of my total HP.
Please stop with the stupid amounts of true damage.
If you were at 30% hp in lane without E/summoners with the actual meta it was your fault dying, everybody can make that 300 hp disappear quickly right now at low lvl.
Also I see that you like to compare Pyke R with Darius R, well, did you know that Darius reset lasts longer or fully resets at rank 3? Did you know that Darius R deals lots of true damage even if the enemy isn't below the required Hp to kill him? Did you know that Darius R can't be dodged or missed because it's point'n click? Did you know that Darius R fully charges his passive and makes him instantly apply 5 stacks of bleed when he kills an enemy with it? Did you know that Darius R full damage is higher than Pyke R and the difference gets bigger and bigger the more the game lasts? Did you know that Darius can gain bonus HP and won't be easily oneshoted?
Dude, you're Ezreal.
How did you not blink away?
So just to clarify. Pykes Ult has nothing to do with true damage, AT ALL. It also has nothing to do with pure damage (which ignores shields too). It is an execute! Just like Urgots Ult. It just shows true damage in your death recap but it is not a true damage spell.
Level 8 pyke with 50 bonus AD should do 320 with his ult
Level 8 Darius with 50 bonus AD from items/runes, with Noxian Might active, and 5 stacks on you, should do 372.5 damage with his ult
Correct me if I'm wrong
Personally I think an ult like Pyke's should do bit less though. Darius has to work a lot harder for it, and Pyke's repositions him for follow ups, and it also does AoE damage to help set up the followup ults.
Not saying that damage isn't all over the place... but maybe you should save that
http://i0.kym-cdn.com/photos/images/newsfeed/001/365/818/183.jpg
So are we just going to completely ignore the 2 different kits? 
This is like arguing as to why all global skillshots dont hit the same*[zombie-brand-facepalm]*
The only good Ezreal is a dead Ezreal.
