Lets pretend that we all agree that it is OK to torture terrorists. One reason we still don't want torture to be national policy is because we can't always be sure that the people we capture are terrorists. We could always torture the captives to find out whether or not they are terrorists, of course, which would retroactively justify the torture. A good option for those who have no problems with sadistic circular logic.
At any rate, this is a similar reason why we shouldn't have the death penalty: we can't always be sure that the person is guilty. If we have 100 men accused of murder and we know 99 are guilty but only one is innocent, do we just kill them all? Of course not. Murdering an innocent man is a greater travesty than letting 99 guilty men escape death by rotting in jail. And of course there are other arguments* against the death penalty that people can make; its more expensive, inhumane, et cetera. This one is sufficient for me.
*Another good argument against capital punishment for people who adhere to Christian beliefs was listed among the Ten Commandments given by God to Moses. It's a complicated one: "Thou Shalt Not Kill". The interpretation of this obfuscated commandment is still an object of enduring confusion for many people, especially Christians who live in Texas.