The Dotharl we meet are relatively peaceful and welcoming precisely because of their massacre of the Hotgo. Exactly when it was carried out is unknown, but the Dotharl suffered heavy casualties in so doing. Sadu recognizes that going on massacres will eventually drive the Dotharl to extinction, which is likely why their aggression was toned down in Stormblood. At present, Sadu is focused on rebuilding the Dotharl's numbers - not going on massacres.

Quote Originally Posted by Gosetsu
[Y]ou are not the most prosperous of tribes. The Mol you mock are few, but so too are you. Why?
Quote Originally Posted by Sadu
...Though a glorious death is to be celebrated, it is yet death. While we kill many, many are killed in turn. In the wake of our battles, the soil drinks deep of the blood of the fallen, their bodies piled high. Not all are born again. Only the bravest. Yet one cannot return unless there is a vessel to inherit the soul, and if warriors die ere they birth children, then that is that. Even the brave warrior who finds a vessel will for many years remain a shadow of his former self. It falls to his elders to endure until he is ready to accept his responsibilities. Ever will we meet our enemies in battle without fear. Such is our way, even when our numbers dwindle. If we die, so be it─but know that we fight to live, not to die. Dotharl train and make ready for the day─but they do not rush to meet it. Not until we have done our duty.
Either the old khan died, or Sadu's priorities have changed from when the Dotharl massacred the Hotgo.

Why full-on massacre of the Hotgo? Unknown. There is very little we know of the massacre itself - only that it happened. Who started the conflict? When? Where was it? Why did it happen? The only thing we know is that it did happen "recently."

There are a number of inconsistencies regarding the Hotgo's own description as well, since Mide is never seen wearing face paint. She's also a skilled machinist, which wouldn't make sense for a nomad of the Steppe unless she left a long time ago and/or the Hotgo still had access to the information in the Enigma Codex, which would enable them to make weapons far more advanced than what we see other denizens of the Steppe using. This would explain the heavy Dotharl casualties, small population, and accordingly more defensive attitude we see them with.

There's just not enough information to determine much of anything at the moment. It may be explained in greater detail later... or it may not.