Depending where you are, misadventure isn't really a euphemism. In England:

Suicide means the deceased did something to end their life, and had the intention of ending their life when they did it.

Misadventure means the deceased did a risky thing and died, but did not have the intention of dying when they did it.

Accident means the deceased was doing a normal activity and died.

Definitions of the manner of death can be nice and clear on paper, but are much harder to apply to the situations of a person's death, so there's lots of overlap between suicide and misadventure. It's often difficult to show intent, and so those deaths sometimes get called misadventure, or they have a narrative conclusion.