Seems odd to me that if someone is having trouble breathing, choking, that they would place a pillow over their face??? That would make matters worse!
I think, being a Justice and from what we read earlier (that he knew what was going on and that he was in their way) he must have known that he'd been poisoned as soon as he started choking!
There is so much time and detail that we don't know about re this case. But I mean even things that an investigation might not bring up. Such as Scalia being aware that they wanted to kill him, certain clues, looks, words that were spoken between them and others that gave him clues that they were ruthless and were after him! This is very important in this case, this little clue that he knew!
Knowing this I would think that as soon as he realized that something was very wrong with his body, he must have also known that they'd gotten to him somehow! Knowing this as you're choking and finding it hard to breath, knowing that someone is soon going to find you there dead, being a Judge, he would have thought quickly! He had to somehow let whoever found him there first know that he had been "killed", not just had a heart attack. Not let them get away with murdering him, no way! He had to think of some way to let others know that he was killed! But how?
All he could think of at the time was to take the pillow and place it over his face! He was choking, he couldn't write a note, there was no writing utensils near by, he was found laying on his back on his bed. Thinking of other cases he may have seen in his lifetime, he quickly took a pillow and placed it over his face!
I remember the first time I heard about this case, the pillow over his face struck out at me as very odd. If someone was poisoned, why would they do that? If someone was choked via a pillow, why would the assailant leave the pillow like that, over his face??? Wouldn't they put the pillow back where they got it? If you're going to kill someone by using a pillow, you are doing that to try and hide how they died and make it look like a suicide.
I think Scalia did it to let us know!