I say this not because I'm pro-death-penalty; I say it because it's a terrible anti-death-penalty argument. There's much better arguments than such word play so simple a child can see through it.
(Cards on the table: My personal position is that, actually, I'm OK with the death penalty in situations where a society can not afford to keep the murderer alive, or the society does not have a reasonably expectation of being able to keep the murderer from murdering again. But I do not merely mean "the society would be inconvenienced", I mean, the normal state through history is that the society literally can not afford to feed a murderer or have a realistic expectation of holding them; a tribe 5,000 years ago of 100 people literally can not afford it. However, in the 21st century, every major civilized society in which we might even be having this debate is very far away from that problem, having both abundant resources to feed and house them, and very realistic expectations that they can indeed keep them away from innocents. Prison breaks today are big news, and still don't happen all that often.)