So,
- If some crimes are punished more severely than others, then the government could accuse its critics of committing these crimes.
- The government should not be able to do so.
- Therefore, no crimes should be punished more severely than others.
But this conclusion is ridiculous. Some crimes should be punished more severely, possibly up to and including the death penalty.
My point is that if you do want to argue against the death penalty, this argument is not it.
The only good arguments against the death penalty focus on the kind of civil society you would rather live in — one that has a bureaucratic apparatus for killing people, or one that does not.
Arguments against the death penalty based on specific crimes are unpersuasive, since some crimes really do “deserve” death, morally speaking: the problem has always been a practical one. Even if some violent (or even white-collar) criminals deserve to die, building a bureaucratic apparatus to administer their deaths would make for an evil sort of society.