I recall reading previously that the States in the US that have the death penalty have higher rates than those that don't.
The cost of convicting and executing the Oklahoma City Bomber was over $13 million.
Failure to deter
Not applied evenly
Human right to live
Actually increases capital crimes
High cost of carrying out capital punishment
You also say 3 years to avoid possible errors, but if you look at the Bishop case in reverse, and he had been found guilty when first tried, you would have had...
Because we're not the USA, we don't hand out death sentences, or meaningless 99 year sentences, we are more civilised. Also the law was changed some years ago, so that life didn't mean life, but that a minimum number of years should be set, to allow for parole where the defendant deserves it.