It has been repeatedly argued that the death penalty violates basic human rights, but it has never been outlawed in America. It is a little ironic that a country that supposedly champions human rights around the world wants the right to kill its citizens. Mercy killing is also a gray area. Is this assisted suicide? Killing anyone you think is suffering? I would worry that the person being killed may not want to be killed. Abortion is also a shady area, because people do not agree on what constitutes human life.
Article 3 says that "Everyone has the right to life, liberty, and security of person." The practices you describe may seem to be in tension with this right. While many might disagree, I would say that the Declaration of Rights was not written with fetuses in mind, so we can take a fairly narrow reading of it when it comes to abortion. Mercy killing, one might argue, does not violate the spirit of the Declaration since presumably it is not taking a life that is not going to end anyway. The death penalty is trickier, except to say that at the time of writing the Declaration, capital punishment was a long-standing legal tradition in many, probably even most countries. So they probably didn't have that in mind, as long, of course, as due process has taken place, and punishments are not out of proportion to the crime.