As I usually join in this twice yearly debate I'll say the same again.
Whilst all of the earlier answers are very interesting they are completely irrelevant.
The answer to the question "...why not just sentence those people to death" is that we cannot because we belong to the EU. There is no need to discuss the morality or practicality of such a measure. The UK abolished capital punishment in the 1960s following votes in its Parliament. Now, even if every voter and every member of both Houses of Parliament - including all the ministers - wanted to restore capital punishment they could not. Capital punishment is forbidden under the European Convention on Human Rights and being a signatory to the ECHR is a condition (through membership of the Council of Europe) of EU membership. So to restore capital punishment the UK would have to withdraw as a signatory to the ECHR, leave the Council of Europe and leave the EU. None of these is likely to happen this side of Hell freezing over.
The moral to this tale - if you want to live in a nation which decides its own form of judicial punishment (among many, many other things) do not belong to the EU.