They lead with the idea that this obviously couldn't be suicide - playing up the role he played in the Litvinenko murder investigation, the supposed suspicions of the US intelligence services, and the fact that some friends and family members shared those suspicions. It took until the 28th paragraph for them to mention that he had been showing signs of depression for a while, and the 33rd to clarify that his wife had apparently caught him trying to kill himself the week before his death. They then explain that doesn't matter because a former Scotland Yard counter-terror officer told them this could just have been the result of secret mind control chemicals. (Seriously.)
Their explanation of his role in the Litvinenko investigation is also just plain wrong from what I can tell. He and another scientist re-calculated polonium-210 intake and lethal dose in 2010 based on the discovery that Litvinenko has been exposed twice, rather than once[1] (which made little difference to the results). He doesn't seem to have had anything to do with the original 2006 investigation, which was carried out by a number of other scientists[2]. All of the things Buzzfeed attribute to his work - the discovery of a lethal amount of polonium, the HPA's public statement, suspicion falling on Russia, and Scotland Yard finding a radioactive trail all over London - in fact appear to predate his involvement by years.
It's been rather disappointing seeing them gain further credibility and attention these past few days.
[1] http://webarchive.nationalarchives.gov.uk/20160613093928/htt...
[2] http://webarchive.nationalarchives.gov.uk/20160613091729/htt...
The rest is a matter of opinion. They obviously have no definitive answer, so it's all speculation. Their point is that his personality changed after these trips.
Maybe I would be more inclined to agree with you if I hadn't seen a Channel 4 documentary on Litvinenko [1] where one of the Scotland Yard detectives recounts lots of strange things happening on their separate trip to Moscow: parts of their group fell ill, they thought they were being followed, and they were driven aimlessly around before they could see one of the suspects.
[1] http://www.channel4.com/programmes/hunting-the-kgb-killers
[1] https://www.ncbi.nlm.nih.gov/pubmed/16410294
[2] https://www.ncbi.nlm.nih.gov/pubmed/17132655