That 2nd verse "tasmad Asaktah.......purushah" you quoted is from 3rd sarga in Bhagavad Gita, unlike the "karmanyEvAdhikAraste..." which is from 2nd sarga. Reason I say that is beacause the answer to your question also lies in the same 3rd sarga itself.
First, Verse 47 in 2nd sarga is saying that we should unite ourselves with the actions instead of uniting ouselves with the fruits of our actions (hence karma 'yoga', yoga=union). I know that the very next thing we question immediately is - what actions, specifically? Shri Krishna answers that very question, but much later in Bhagavad Gita (verse 24 in Sarga 16 to be precise ^).
However, I would rather orient you to another statement Shri Krishna makes within Sarga 3 itself, since you can use this statement as your guide/measuring stick/filter that ANY action must first pass through before applying "karmanEvAdhikArasthe..." shloka. Using this filter will quickly address dour doubt/question here.
Shri Krishna states in verse 9 in Sarga 3:
यज्ञार्थात्कर्मणोऽन्यत्र लोकोऽयं कर्मबन्धन: |
तदर्थं कर्म कौन्तेय मुक्तसङ्ग: समाचर || 9||
yajñārthāt karmaṇo ’nyatra loko ’yaṁ karma-bandhanaḥ
tad-arthaṁ karma kaunteya mukta-saṅgaḥ samāchara
Meaning: Work must be done as a sacrifice to the Supreme Lord; otherwise, work causes bondage in this material world. Therefore, O son of Kunti, for the satisfaction of God, perform your prescribed duties, without being attached to the results.
That is the key word, sacrifice.
Now apply your example: if the person is doing something because he likes it, where is the sacrifice?
i.e. in your example the person is already wedded to the end result of his actions - the killing (ending of a life). Why? because he likes the end result (killing/ending of a life) as you said. When emotions are the driving force, there will be no sacrifice.
The right example that ticks all these boxes (incl. verse 24 in sarga 16) is the killing done by the Judicial/Legislative Executioner, who is indifferent to the end result (killing of the sentenced prisioner) and does his act of execution in a purely detached fashion. Not because he likes killing, but because shAstra (Penal Code in this case) requires him to do that very act for the sake of civilization.
When you like the end result, that "karmanyEvAdhikArasthe.." verse does not apply because the 'liking' had already transaggressed that verse.
In case you say that what you meant in the example is that the person likes the act of killing as an adverb, vs. as an end result, then let me challenge you to think through it a bit more - what does the murderer really like, the act of choking, or the act of choking someone because it does something to that someone? (i.e. driven by the end result). A psychopath will not like choking if that act is not doing something to that someone.
^ Verse 24 in Sarga 16:
तस्माच्छास्त्रं प्रमाणं ते कार्याकार्यव्यवस्थितौ |
ज्ञात्वा शास्त्रविधानोक्तं कर्म कर्तुमिहार्हसि || 24||
tasmāch chhāstraṁ pramāṇaṁ te kāryākārya-vyavasthitau
jñātvā śhāstra-vidhānoktaṁ karma kartum ihārhasi
Meaning: Therefore, let the scriptures be your authority in determining what should be done and what should not be done. Understand the scriptural injunctions and teachings, and then perform your actions in this world accordingly.