The inherent risk of killing the innocent:
Since 1973, more than 165 people who have been sentenced to death have been exonerated. If they were not exonerated and their death sentences were imposed, our country would have killed 165 innocent people. As long as the death penalty exists, we run the risk of killing the innocent.
The death penalty does not deter crime:
Many proponents of the death penalty believe it deters criminal activity. This is not proven. In fact, murder rates are higher in the south where the death penalty is mostly imposed compared to the northeast where it is imposed the least.
It is more expensive to sentence people to death:
When the death penalty is a possible outcome of a case, the stakes are higher; more attorneys and experts have to be hired on both sides, trials are longer, and there are more appeals. The state typically pays for all of this.
Additionally, while many states still sentence people to death, the penalty is rarely imposed these days. The result is that defendants serve a life sentence but at the cost of a death sentence, which is much higher. If they were sentenced to life in the first place, the state would avoid the extra costs.