Atheists do not believe in an afterlife. Their focus is on this life and making it the best possible life. For some people, this may mean living their lives as they see fit, and it may or may not include a duty to care for and respect others.
Peter Singer is a moral philosopher, who popularised the term speciesism. Speciesism is the idea that humans have greater moral rights than other living beings, based simply on the fact that they are human.
Singer opposes speciesism, as he believes that all living beings should be treated equally. He believes that to treat humans differently to other animals is the same as treating one race differently to another. Singer therefore fights for animal rights and promotes veganism.
Speciesism — the word is not an attractive one, but I can think of no better term — is a prejudice or attitude of bias in favor of the interests of members of one’s own species and against those of members of other species.Peter Singer, Animal Liberation 1975
Many people believe that quality of life is more important than the right to life. If someone has no quality of life, then is there any reason for that life to continue? If a person is suffering and in pain, some people would argue that they have the right to end that life and opt for euthanasia.
Similarly the same applies for abortion. If tests have shown that a baby is likely to be born with severe disabilities, some people may argue that to end the pregnancy may be the best option.
It is difficult to define exactly what 'quality of life' means. People who are living with severe disabilities have very different viewpoints on their own quality of life. Quality of life is different for each individual, as is their ability to deal with pain and suffering.
Many religions and faiths would take an absolutist approach when it comes to decisions on life and death. An absolutist approach means that actions are viewed either as being always right or always wrong.
People who have no belief in an afterlife and believe that it is only this life that matters, may take a situationist or relativist approach to decisions regarding the right to life and death.
A situationist approach considers each individual situation. With this approach, decisions are based on what is best for people involved in a particular situation. As humanists have no doctrines or set teachings, each person will have their own approach towards decisions on life and death, however most humanists take the situationist approach.