It's not that there aren't certain people I feel the world would be better off without. I think there are such people. But it's not up to me, us, or anyone to determine this.
First, the death penalty is applied unfairly. This is true of pretty much ALL criminal penalties, but the death penalty is final, and there is evidence that the bias against poor and minority people, especially men, may be particularly severe in death penalty cases.
Second, the evidence of its efficacy is sketchy at best. As a statistician, I can say that is almost impossible to really evaluate the efficacy of capital punishment. It's difficult for ANY punishment scheme, but especially so for one that is applied rarely and unfairly.
Third, taking life cheapens life. I recall reading (I think on Twitter) that a woman asked her kid if she (the kid) would favor the death penalty for Hitler. "No" said the child. "People shouldn't kill people". Out of the mouths of babes.
Fourth, the death penalty isn't cheap. One possible (although not, to my mind, very good) argument for death instead of lifetime incarceration is that it is cheaper. But it isn't.
Fifth, and finally, people make mistakes. Even without bias, even with every defendant getting a good defense and a fair trial, people make errors. It is better that our errors be fixable, to the extent possible. If we jail someone and find out our error we can (although we don't always do so) release the person and try to make some restitution. If a person is dead, there's nothing to do except say "We goofed".