^Lol I didn't know we had a Wahabi on a Qur'an alone site. Beware of these type.
Gnosis, if you are to kill someone for spreading mischief then you can kill them for anything. Going around fommiti g armed robberies is spreading mischief. Braking into houses and stealing is spreading mischief. Sleeping around before marige is spreading mischief. I think you have just picked what you personally find distaetefull and made up a death penalty for it.
Plus The only thing I might agree that Quran is saying what you are saying is on murder.
"On that account: We ordained for the Children of Israel that if any one slew a person - unless it be for murder or for spreading mischief in the land - it would be as if he slew the whole people" - al-Qur'an 5:3
The spreading of mischief is understood in the proper context as the type of mischief that would be equally as serious as murder.
In the ayat of Allah, He is discussing how serious of a crime murder is, as He says the murder of one person
"would be as if he slew the whole people". He gives exceptions for the slaying of one who has comitted murder (a very serious offense) and for spreading mischief.
The crimes that you described, which were robbery and theft would not be considered part of the "spreading mischief", and they have their own punishments within al-Qur'an:
"[As for] the thief, the male and the female, amputate their hands in recompense for what they committed as a deterrent [punishment] from Allah . And Allah is Exalted in Might and Wise." al-Qur'an 5:38
^The Arabic word "yad" for hand is used in the Qur'an both literal and metaphorically as per ayat 27:12 is literal while ayat 5:64 uses it metaphorically. Metaphorically speaking, cutting of the hands would mean cutting off their resources i.e. income etc to repay the debt of whatever was stolen and any other harm done. Dependent upon the crime, both understandings can be used i.e for repeated convictions of theft crimes, the literal understanding can be applied to prevent them from repeating it again.
Murder is a very serious crime as it permanently terminates the life of one, and permanently effects the lives of others i.e. family members and loved ones. It is the fact that murder cannot really be paid back and it endangers the lives of others in society and if the victim and society does not choose to excuse their crime (which they have a right to do), then the only repayment for the victims life is the life of the one who commited the crime. Rape permanently brings trauma to the victims for the rest of their lives and that cannot be paid back. Their life will forever be one of that has that dark memory and fear that it won't happen again. It phychologically effects people permanently. This is especially for virgins who get raped. You cannot pay back virginity. Rapist also threaten the livelihood of the society with fear, STD's, etc. Child molestation is equally as serious as a crime for the same reasons.
It is in these ways that Rape and Child molestation are equally as serious as Murder and can be understood as being the
"spreading of mischief in the land" that ayat 5:3 mentions. The "in the land" being part of the ayat stresses that this type mischief is one that also threatens society, which Murder, Rape and Child molestation does.
As the punishment for Murder is death, so should it be the punishment for Rape and Child molestation, for they are equally as serious of a crime as Murder, so the punishment should be equal. This would then make both ayat 5:3 and ayat 2:178 agree.

Hope this helps.
