If there were no false positives in the justice system, your argument might be reasonable. There's always a chance a person will be mistakenly given life without parole. They can be released, an executed person cannot. I guess it depends whether a) you have more trust in the justice system than evidence would support, or b) you think the false positives (dead innocents) are a reasonable price to pay.

Additionally, people can and do kill themselves in prison: there is a way out of serving a full-life sentence that doesn't involve judicial killing.