This is an interesting post with some ideas worth discussing.
I think the death sentence is wrong because anyone who is willing to commit a heinous crime is not rational.
Do you mean that no one 'in their right mind' would kill? What about someone who reasons that the only way for them to protect either themselves or their loved ones is to kill another or a group of people? If I have reason to believe that my next door neighbour is planning to break into my flat and is himself 'not in his right mind' (and so not receptive to reason) and I kill him to prevent him from doing so, am I acting rationally?
There is also a problem in applying this idea of 'irrationality' in that someone who has always appeared to act rationally (and has never given us a reason to think otherwise) might suddenly do something 'out of character'. Are you saying that the act of killing is 'out of character' for all people and so any instance of this act is to be taken as being 'irrational'? Would you be willing to admit that anyone who doesn't kill is entirely rational? Is the act of killing the only thing a person has to do in order to be irrational?
Lastly, could it be argued that anyone who is not in their 'right mind' is a danger to others and should be focibly removed from society, perhaps even through being put to death? I admit that this is an extreme way of looking at it but it is a possible justification nevertheless.
There is a breed of sharks that their brain produces a sexual endorphin when they hunt
It has also been found that wasps experience a similar degree of pleasure when they sting someone; however, we are talking about sharks and wasps, not people. So far as we know sharks and wasps are not engaged in the activity of 'society' (at least not in the way we are). We may share physiological attributes with other animals but are distinct from them insofar as we are able to recognize these attributes and how they impact on social activity.
What if we find the chemical imbalance in peoples' heads that causes them to be wicked and develop a medicine to regulate it?
We are already attempting to do this. Anti-psychotic drugs, electro-shock therapy and 'behavioural modification' are ways in which society attempts to refine what it deems to be unmanageable (if not necessarily 'wicked') behaviour.
Also, if this wonder drug were to be developed, who would determine which behaviours are to be deemed 'wicked'? Alongside this, if a person were to take such a drug and then commit murder, it could be argued that they are no longer a 'wicked' person so there must have been a wholly rational basis for their act.
As it stands I really don't think we could ever find a cure to people like that and to know if they needed the medicine they would already be condemned
You clearly already see my point. However, this does not stop society from attempting to control what is deemed to be irrational behaviour.
Put simply, while there are many who are either not capable of controlling their actions or not capable of understanding the severity of them, there are countless others who take a measured, rational stance towards what we understand to be criminal behaviour. If everyone who commits murder is 'insane' then what of other criminal acts? Where do we draw the line, cease to medicate and begin to attribute responsibility?