First, there's the human issue. A fetus that can be legally aborted is not advanced enough to be considered a fully-fledged human. They can not feel pain, they have no personality, they have no memory, because they have no nervous system. They are a bundle of cells with the potential to become a human. In opposition to this, a man or woman being killed, regardless of their crime, is a human being. They have memories, they have hopes, they feel pain, they have mothers and fathers, and maybe children that love them. It is easier to empathize with an adult, even a monstrous one, than it is a ball of flesh.
Second, there's practicality. Whatever you think of abortions, take away that outlet, and you find yourself in situations like we did even 50 years ago, with women visiting back-alley "surgeons" wielding coat hangers, and often contracting tetanus and other illnesses, since these guys had no oversight. You may not like abortions, but recognize that regardless of legislation, women are going to seek them, and it's better to have regulations. So, I can protest the death penalty all I want to save a life, but if I want to outlaw abortions, I'll have more blood on my hands.
Then we could also point out that many of the people sitting on death row are innocent, or even more so, repentant. A man can, contrary to popular belief, make something of himself in prison and turn his life around. Many have done it. Hard to do when your dead. Also, how can we execute a man or woman without 100% certainty that they are guilty? We put their lives in the hands of twelve people who don't want to be there and just want to get the trial over with and go home. There will be and have been mistakes. Those mistakes aren't punishment. They're legal murder.
Personally, I don't support abortion, but I do support its legalization, because the alternative is worse. I also do support the death penalty, because for all the mistakes, we have limited tax money, and better things to spend it on than keeping prison inmates alive. I just demand that the legal system be better.