This has always been a misperception by Catholics and non-Catholics alike. Here is what the Catechism of the Catholic Church says about the death penalty:
“Assuming that the guilty party’s identity and responsibility have been fully determined, the traditional teaching of the Church does not exclude recourse to the death penalty, if this is the only possible way of effectively defending human lives against the unjust aggressor.” Catechism 2267
Most people who kill others had to make a conscience decision to take that life and therefore willingly commit mortal sin. That is why the Church will allow the death penalty under the circumstances mentioned above.
When said offender is effectively incarcerated for life, where is the risk to other human lives?