How can anyone support the death penalty,?

You've just come up with two uselessly vague arguments that even imply nobody should be convicted of any crimes at all, that nobody should be punished at all. 1) "Law enforcement can be corrupted and/or incompetent," and 2) "Many people are wrongfully convicted for crimes they didn't commit." Neither one of those two statements have anything to do with why it is okay to convict a person and sentence them to life in prison without the possibility of parole but not to execute them. Indeed, even when a person's conviction means no jail time at all but merely some kind of fine, both of the conditions which you described might be applicable. Or might not be.