I'm not so sure. Big brain not the same as 'intelligent'.

A story: every year a Japanese dog food company goes to a certain island lagoon where dolphins migrate to reproduce. They net and kill thousands and take them away.

Each year some dolphins escape, inevitably. They swim around for a year, and then the next year come back (with many other dolphins) to the lagoon, and likely get netted and killed.

Didn't they remember what happened? Didn't they communicate it to other dolphins? Don't they care?

I have to conclude either

   Dolphins are not smart enough to remember from year to year,

  or Dolphins can't communicate with other dolphins in a sophisticated enough way to say "danger! evil here!" which is pretty basic,

 or Dolphins are dicks and don't care if other dolphins are killed,

 or heck, maybe Dolphins, like Klingons, enjoy the challenge? "Today is a good day to die!"

I conclude that, in none of these scenarios, is there much wrong with killing dolphins for dog food. Not in a 'killing our sentient brothers!' way.