What is the problem with 'observations'?
Darwin looked at something and took note of them, that is an observation.
I don't think he started at the end and worked to come up with something that fitted what he wanted.
He spent many, many years studying, investigating and observing.
He was originally a strong believer in God being the designer of all things.
His views changed over time. The final push for him was the death of his daughter Annie in 1849, he then lost all faith in a benevolent God, coming to the point, when asked, where he said that agnostic would be the best description of his beliefs.
Given the time in which he lived, I think it was a mixture of his religious upbringing (unitarian,anglican), where science was at the time and the human mind reaching for answers to observed traits (for want of a better word) that he wanted to be able to explain in scientific terms.
His first book The Voyage of the Beagle published in 1839 was a travel memoir and also showed the great extent of his powers of observation in the fields of geology, biology and anthropology. It was from this publication that he became famous. Although I think that what he covered here was what came to be known as macro-evolution, there were hints in it that he would later develop into his theory published in 1859 in The Origin of the Species which would encompass micro-evolution.
In summing up, I think he really believed in the conclusions he came to and that it developed over time, not as has been suggested in the OP that he decided on a conclusion and sought things to back it up.
Would have been nice if that could have been proved as the case, but even if it had I think the whole common descent thing is too entrenched in the human psyche to be dented by it.