This perk is bugged in multiple ways. One, sometimes when killing unaware humans, a heart is not harvested. Two, even when a heart is harvested, HP does not always increase. This may be related to the Treacherous flaw, which reduces Base HP by 25% (which should not affect Max HP, but this may just be bad wording), but in that case the heart should still give +1.5 HP instead of 0. 

Update: I don't know if related, but after the patch I actually LOST HP after collecting a heart - from 678 to 677. What's going on?

Playing it now and Treacherous negated Serial Killer as soon as I got it. I'd never have taken Serial Killer if I knew it would be a waste.

+1 i think i have this issue too. trying to start a psychopath stealth build and my hp don't seem to increase but decreased 

The Serial Killer perk is bugged for me in the same way. Having taken a total of -40% base health in anticipation of this perk, this bug breaks my character and I am going to wait for this bug to be fixed before continuing to play.

I am having the same kind of bugs with the serial killer perk. It went up by 2 , then back down by 2 after two kills. Then it seemed to be working for about 6 kills in a row, I saved ,re-loaded the save file, and it reset my max health back to the starting point as if I had not made those kills. Obsidian please fix this bug. I made an account just to request this bug fix. 

Also does this for me. The unaware enemies part doesn't work at all. I did the math. I have a friend who has a character at the exact same level with both treacherous and sickly. I have 230 hearts. Are hp is practically identical. Just a few points different. The math simply does not check out. 

Same here. just picked up the perk, killed at least a dozen humans with sneak attacks.  completely stealth, no engagement. I have only two hearts in my inventory.