Question about Lethality

I've been reading A LOT of topics regarding the newest boogieman to league of legends and I genuinely don't understand a lot of things about it.

From what I can tell, Lethality provides a good deal of flat armor pen that also scales with % pen. I'm going to try something here so forgive me if my math is off.

+ + = 50 lethality and 185 AD

Armor pen = LETHALITY x (0.6 + 0.4 x Target's level / 18) | / Armor pen = 50 x (0.6 + 0.4 x 18 / 18)

Armor pen = 50 x (1)

With this in mind look at this build.

HP = 1800 Armor = 50 MR = Irrelevant

vs

vs This build

HP = 1000 Armor = 210

Assuming that base stats are not involved in calculations and that both champs are 18 but don't have any other items.

The AD attacker would hit build 1 for essentially true damage dealing 185 hp per auto or around 10.3% of his total hp.

The AD attacker would reduced build 2 armor to 160 and deal 71 hp per auto or around 7.1% of his total hp.

Now to compare the cost:

AD attacker = 9250 Gold Build 1 = 8550 Gold Build 2 = 8150 Gold

Final thing to note is that all of the Lethality items all build from which has 10 Lethality until completed, none of them provide hp, and only one of them provides any kind of protection from cc.

Now I know that this is mostly in a vacuum and thus lacks contextual information needed, (like base values, abilities, and passives) but I chose to forgo bringing in specific examples because it would miss the point. What is it about lethality that's breaking the game?