However, there are somethings you may have missed.
You said Heimdall insulted Drago without provocation, yet there was provocation. "Drago: I fear your brother has made an unwise decision." Now whether or not intentional, Drago insulted Heimdall's brother and leader. It was slightly mitigated by the fact that Drago at least had the sense of mind to whisper that comment. However, that was not enough to reduce all the offenses that insult carried they are:
1. Drago essentially in Heimdall's mind called his brother decision foolish, the same thing Heimdall called Drago judgement of K'Warra without all the information.
2. Drago said it in public, even though at a whisper, others could have easily overheard. Nothing prevented Drago from holding his tongue and talking to Heimdall in private later.
3. Drago's concern about Vidar's decision should have been brought to Vidar not to Heimdall. That action could easily be seen as Drago attempting to cause Heimdall to doubt his brother's judgement. This easily caused Heimdall to react a little stronger then normal. And was a leading reason for Heimdall to choose the word "foolish" instead of something else.
Now onto the rest of Heimdall's comments. Heimdall was not trying to teach Drago anything, then why those comments? The answer is in those who are listening to the conversation that could very well need a lesson.
When Aidan spoke in support of Drago, Heimdall used a simple example not as an attack on Drago but as a way to easily give a lesson on keeping an open mind. He used slavery since that is what Aidan spoke about, it had nothing to do with what Drago had said or been saying.
Heimdall then leaves without another word not because it is of little consequnce but because of the amount of work he has to do. He meant only to take a short break, but instead it was much longer then he had planned.
Now a couple of things about slavery, Heimdall doesn't like slavery, nor do any of the Aesir. Also he was about to keep K'Warra in a cage until Vidar said to release K'Warra and the other two.
Oh, just so you know Allessa and Hal are going to have a hard time convincing the Aesir to take her along. She doesn't seem to be very useful and doesn't seem to take hardships as well.
"Dulce bellum inexpertis."
Official Forum Expert on Norse Mythology
As Odin says in the Hovamal:
"Praise no day 'til evening; no wife 'til on her pyre; no sword 'til tested;
no maid 'til bedded; no ice 'til crossed;
no ale 'til drunk."