La howla wala kowata Ela, Billah.
Islam is the religion of Allah, the divine religion of God.
Thus, when people claim to be of it, then leave it, and then spread mischief callers others away from it, death is better to them then continued life.
It's a mercy, for If they continued, their place in hell would be lower, and lower, and lower.
We judge law by the belief of an afterlife 100% without a shadow of a doubt.
Thus, death is not a "bad" thing If it is done to pervent chaos.
Let's say, a person left Islam, and started calling people away from Islam, through the eyes of Islam, a person is calling people to jump into hellfire, now, would it benefit mankind that this person live or die?
It would benefit both this person and mankind, of course, that person is first judged and as well as talked to obviously to try and convince him/her to return to Islam and to change their ways.
Now, again, based on the FACT of an afterlife, exactly as it is described in Islam, everything inch by inch.
Is it in anyway strict? too much?
You see, people who are not Muslims, judge with the belief of this life and no afterlife, or a possible afterlife but still with doubt.
Thus, their judgement is based on a belief, and ours is also based on a belief.
The belief of an afterlife, the belief of eternal and temporary punishment in hell and to kill the devil when he starts calling people openly.
Because (since in Islam, we judge based on Islamic beliefs) any person who leaves Islam and then calls others away from it is either: Extremely misguided (doesn't know anything about Islam) or Highly influenced by the devil, arrogant and ignorant, wanting a desire for this world and caring not for his/her afterlife.
That belief is there because Islam judges by the belief that Islam is 100% the truth.
Now, based on THESE beliefs, is the punishment harsh?
You must learn that you are not all-knowing, infinite in wisdom. Only Allah is.
Until you understand that, you will always think "Oh why do these people follow this ridiculous religion" while in truth, because of their wisdom, they SEE what you do not.
and what is it? Exactly what I explained to you above, If you were wise, you would not need my explanation on why Islam is fair and just because it believes in an afterlife and thus all judgements are based on that belief and the rest of Islamic beliefs.
Now, there was no afterlife, If God didn't exist (May Allah forgive me for that is an If that is beyond reality) Then your opinions about judgements in Islam would be understood and agreed upon.
You see and are enlightened? Or did Allah increase the disease in your heart and cover your eyes and ears with a veil that such a clear explanation walked right by you?