Are you talking about killing as the sentence imposed after a juridical process or are you talking about gunning a person down with no trial?
I don’t believe that the latter is in any way justifiable.
As for the former, the Catechism teaches us:
2267 The traditional teaching of the Church does not exclude, presupposing full ascertainment of the identity and responsibility of the offender, recourse to the death penalty, when this is the only practicable way to defend the lives of human beings effectively against the aggressor. "If, instead, bloodless means are sufficient to defend against the aggressor and to protect the safety of persons, public authority should limit itself to such means, because they better correspond to the concrete conditions of the common good and are more in conformity to the dignity of the human person.
If all the conditions listed above are met, then killing an abortionist, after a conviction, could be licit.