To declare any year as 'the worst year in history.' we need to look beyond a Eurocentric view, and beyond West Asia.
I havn't the background knowledge to check what was happening in North and South America, or Southern Africa or the Australian continent. Although I imagine that the Icelandic volcanic eruption would certainly affect all the northern hemisphere, But I can comment reasonably on Eastern Asia especially China. This area too was affected by both climate and political events. The authors of this article suggest that the political events in that year (and that time period) around China were related to climate events.
See, "Natural Disasters in the History of the Eastern Turk Empire,"
The article Abstract reads, "This article analyzes the effect of climate extremes on the historical processes that took place (AD 536, 581, 601, 626 and 679) in the Eastern Turk Empire (AD 534–745) in Inner Asia. Climate extremes are sharp, strong and sometimes protracted periods of cooling and drought caused by volcanic eruptions that in this case resulted in a negative effect on the economy of a nomadic society and were often accompanied by famine and illness. In fact, many of these natural catastrophes coincided with the Black Death pandemics among the Eastern Turks and the Chinese living in the north of China. The Turk Empire can be split into several chronological periods during which significant events that led to changes in the course of history of the nomadic state took place: AD 534–545—the rise of the Turk Empire; AD 581–583—the division of the Turk Empire into the Western and the Eastern Empires; AD 601–603—the rise of Qimin Qaghan; AD 627–630—the Eastern Turks are conquered by China; AD 679–687—the second rise of the Eastern Turk Empire. The research shows that there is clearly-discernable interplay between important historical events and climate extremes in the history of the Turk Empire. This interplay has led us to the conclusion that the climatic factor did have an impact on the historical processes that took place in the eastern part of Inner Asia, especially on the territories with a nomadic economy."
And the article's authors conclude: "Though much discussion is going on at present among researchers concerning the problem of the influence of climate changes on nomadic society, we hold to the opinion that the historical events that took place in the Turk Empire in AD 534–551 and in AD 679–687 developed in a line parallel to the unfavorable changes in climate which may be considered as an additional factor that led to changes in the regional situation. Historical sources show that in the year AD 534 the Turks had already become stronger—a fact that happened prior to the climate extremes, but the discontent of the Turks with Emperor Gaozong’s (AD 650–683) policy grew during the whole period of his reign and it is this that led to the rebellion of AD 679. Nevertheless, the sharp changes in the economic and political situation in the Turk Empire in AD 581–583, in AD 599–603 and in AD 627–630 totally coincide with the extreme changes in climate in these periods, a fact that may be looked upon as indirect proof of the interplay of historical processes in the region with climate dynamics."
For anyone that is interested in examining this issue further there's a need to establish what was happening in the Southern hemisphere, and I suggest in India.