Learn English – a word that means something like a “seal of approval.”

I'd say "the tone of the trip changed."

Deadly is generally taken to refer to humans, and so does not need a qualifier. In fact, I can't actually think of a word that would be comfortably and effectively used in the way you want to use it.

Also, we usually say "deadliest" instead of "most deadly."

It is understood that we are talking about people in such cases, unless we specify something else. Therefore, if you want to indicate it killed a lot of alligators, you would say something like, "For the alligator population, it was one of the deadliest events...."

If you still feel the need to emphasize that you're talking about human beings, I would suggest you say "in human history" rather than simply "in history."

Addendum to address the new final sentence of your edited question:

I really can't come up with a word that works for your purpose. If you want to specify that you are just talking about how many people died, I suggest you restructure the sentence. Here are a few possibilities:

The hurricane killed more people than most other events in recorded history.

The hurricane was one of the deadliest human tragedies in history.

The hurricane was an event more deadly to humans than almost any other natural disaster in the history of the planet.

And there are many more.