How will humans will go extinct?
Will we? Yes probably...
Well... It is very likely that humanity will go extinct at some point even if we colonize other planets.
Things that could cause extenction that I can think of are:
Alien colonizers (if interstellar travel is possible)
Genetic Collapse is one possible answer.
Mutations tend to accumulate in a population. When the mutations reach a critical level the population can not longer reproduce successfully and extinction results.
There is evidence that a population of elephants on an island off of Alaska experienced extinction due to genetic collapse.
Both the Cro-Magnon man and the Neandertal man during the ice ages had larger brain capacity than modern man. The Neandertal was most likely much stronger physically than modern man. It seems there has been a lost of genetic strength, in modern man.
There is evidence of genetic collapse in the human genome. Modern man is more likely to have more eyesight, impaired function of the appendix and tonsils ( valuable parts of the immune system) lactose intolerances and a greater occurrence of cancers.
There is a possibility that sometime in the future these genetic mutations will accumulate to the point that the human population will become extinct.