I can answer this from the US angles, which largely mirrors the European thinking. Stats from this Wikipedia entry
- The justice system sometimes fails. There are people unwilling to accept a system that could execute an innocent person. 157 people sentenced to death in the US have been exonerated since 1973.
- Death penalty cases largely mirror life sentences anyways. The average time on Death Row is about 15 years. 20-to-life is not that far off.
- There's no truly "kind" way to execute someone. While lethal injection has been favored of late, several high profile problems, as well as difficulty in procuring drugs have cast doubts on the ability of some states to execute people.
I should note that even attitudes towards life imprisonment may be changing. Sweden has a maximum sentence of 18 years, with 10 years being the new default.