The very idea of emptying your mind and allowing yourself to react to the flow of battle without unnecessary thoughts is what makes UI the DB equivalent of Mushin. Whis does not lose stamina or become hot/sparkly/tired while using it. It's a state of mind, not a physical power-up, or a physically draining state of the body. That's what he taught Goku.

As for why Goku becomes all hot, why he only gets into this state after getting utterly owned, and why he only does this after absorbing a ton of energy or seems to drain "stamina", it's anybody's guess. They haven't explored this in great detail. Whis is guessing that Goku is breaking out of his shell, whatever the fuck that means.

What we do know is that UI doesn't, and isn't, the above physical attributes. Whis has demonstrated this already.

Regarding intent to kill, can you find me somewhere where Bruce has actually advocated this?