I was thinking about the Hitman character of Agent 47 and I thought if in the future genetic engineering could be used to create a person with no emotions and feelings at all. How could this be achieved? Mutations in the limbic system of the brain maybe?
All it takes is a disruption to a small ribbon of nerve tissue In the middle of the brain. Side effects are inability to prioritize, inability to recognize or respond to normal social cues, and unpredictability because they will act on impulse. Memory formation is also governed by emotion, so teaching them anything and having them remember anything that was not immediately critical would be an issue. Emotionless hitmen are unreliable hitmen.
I know this because of a brain injury I live with.
Emotions are the basic operating signals of the brain, you can no more have complex build a brain without emotions then you can have a windows program without any binary. A brain that cannot process pleasure cannot learn. Pleasure is how the brain builds correlations between events (engrams). You could create a reduced spectrum of emotions but eliminating all emotions is impossible if you want them to be able to do anything.
There are people that cannot consciously or subconsciously process emotions, Alexithymia, in some cases even lack some "higher" emotions, but they still possess some emotions, however they often exist only at the nonconscious level which may be the closest to what you want. Interestingly this often makes these individuals vulnerable to addiction, the brain can still feel pleasure but they are not consciously aware of it which may make addition easier since it is bypassing some the major ways the brain regulates behavior.