Why You Should Get This Perk ASAP
You should get the first perk under Cool attribute Cold Blood skill tree: Cold Blood as early in your game as you can, I would even go as far to suggest spending your very first perk point on it.
The perk by itself doesn’t offer much, giving you a 2% movement buff for 10 sec after defeating an enemy, and at rank 1 can only stack 1 time. But the main reason to get this perk is because this is almost the only way you can gain exp to your Cold Blood skill tree in normal game play.
So just put 1 perk point in it and forget about it, and as you defeat enemies you normal would, your Cold Blood skill tree will level up in the background. You will get back the perk point by the time it hit level 4, in fact you get more perk points at level 5, 9, 10, 11, 13 and 17, along with other various bonus from this skill tree by just having the Cold Blood perk running in the background.
If you got this perk early on you should have enough exp to get the skill to level 20 no problem. But one additional tip if you need some extra: if you defeat any enemy non-lethally, you can go and finish him off with a lethal attack. The Cold Blood perk will pop again and in effect get double exp for the same kill (If you are actually undetected at this point, you would also get Stealth exp).