So what is the deal here? Did Ronan commit all of that genocide in and prior to Guardians Of The Galaxy off his own bat or was his alliance with Thanos intended to be part of the Titan's cull on the universe's population (as I thought that his prime task during and before the film was to retrieve the Power Stone)?
Considering that it seems apparent that Ronan was physically responsible for the murders of Hovat and Kamaria, Drax seems to have an axe to grind at Thanos for the same reasons.
Take the example of when Thanos meets Gamora. The people are split into two groups, and on Thanos' command, Thanos' henchmen shoot one of the two groups. Thanos did not technically kill Gamora's people himself, he "merely" led others to do so (I'm aware that this is an understatement - but it's technically true).
Thanos can't physically kill every single inhabitant (of the unlucky half) of every planet himself. He must have others who operate on his orders.
It seems similarly unlikely that Thanos is personally present for every group of people where half of them are killed. Again pointing to the Gamora scene, the people in that square were by no means the entire population of the planet.
And it seems counterproductive/nigh impossible for Thanos to only sort and kill groups one by one. Word would spread people would go into hiding, and how would you tell a "lucky survivor" from someone who hasn't been through the culling yet? Many issues arise if you assume that Thanos' semi-extermination of a planet's populace is a slow process.
The most logical explanation is that Thanos' henchmen - presumably led by a lieutenant such as Ronan, Ebony Maw, ... - operate the kill command at around the same time all across the planet. I'm not saying it's exactly at the same moment, but sufficiently close that word can't spread.
Now consider that the victims of this culling, who are attacked by a lieutenant when Thanos is not present, are liable to think that this lieutenant is the one calling the shots.
This is likely what happened for Drax. He saw Ronan and "his" henchmen show up and kill his family and thus blames Ronan for making the decision that kills his family.
Even if Drax knows about Thanos, his instinct will still be to blame Ronan because he saw Ronan call the shots.