Raping, sodomizing, sexually assaulting women and children is barbaric to a more severe degree than capital punishment in my humble opinion. 

Housing the the offender is costly, the offender could escape or be freed by a liberal parole board only to offend again (possibly even the same victim) costing more money to prosecute as well as leave the victim without a sense of security and closure etc...(since most sexual predators threaten victims or the victim's families with death if the report)  

Putting these...things (I can't see them as human or animal) to death ends the threat to society and eases the constant fear of the victims of a repeat offense.  If a vicious dog attacks anyone, especially repeatedly, it is euthanized.  Sexual offenders, especially repeated sexual offenders are worse than vicious, biting dogs.