1) The death penalty is more expensive
2) People sometimes are wrongly convicted, happens, will always happen, another point: Law is different, depending on where the trial happens, what state you are judged in, sentences differ. There is no way the death penalty can in any way be "fairly" handed down. (Law is not mathematics, it's a social science)
3) Does it keep people from committing crimes? No obvious answer, likely not
4) There is a racial factor involved, black killing white more likely to get the death penalty than black killing black for example.
5) There are no good or bad people, that might be what religious people think, but you aren't born a criminal, nor is it simply your well informed free choice to become one. What if I had grown up in a low income low educated black community with high criminal and incarceration rates, without a father and a single mother struggling to provide for her children? I am privileged and I know it, so even if someone does something hideous, shouldn't society not be one of revenge, but one that while it punishes, also tries to lend a hand and tries to give support to a poor soul? (the American justice system is not working well in that regard, solution is more therapy, psychologists, less life-long sentences, no minimum sentences, better education, more social workers, et cetera, not death penalties...)
[1] http://www.deathpenaltyinfo.org/costs-death-penalty
[2] http://www.theguardian.com/world/2014/apr/28/death-penalty-s...
[3] https://www.law.upenn.edu/live/files/1529-nagin-full-reportp...
[4] http://www.theatlantic.com/politics/archive/2014/06/race-and...
We value life according to some simple factors, how intelligent is it, how close is it to ours, how do we estimate its perception of its surroundings, its "degree of consciousness" so to speak, et cetera, in that regard killing an animal or a human being is different. It's also different whether you kill an ant, or an elephant. That's why killing an animal unlawfully does not have the same legal consequences as killing a human being. Your comparison is not really relevant and fair. Regarding abortion it's the same, we are not talking about aborting fully grown human beings, I am pretty sure we are not talking about aborting fetuses one month before birth, more likely 6,7 months before birth in most cases... and even then I see how it is a moral dilemma for some people.
And wars should also be the absolute last course of action exactly because it kills people. And you see the consequences, does killing abroad make the United States safer?! In a lot of cases I doubt it, it fuels hatred and radicals... does killing criminals help anyone? let me summarize: it costs more, there is a racial bias, there always will be innocent among the criminals sentenced to death penalty, you are not a criminal out of free choice, violence fuels violence, it's likely not to deter crime, ...