I kinda sorta have this thing in my own comic, which pretty much starts with not one, but two false protagonists XD the first one is killed within the first few pages, while the second one makes an appearance right after, buuuuut... is not the main protagonist of the story at all.
So yep, I think false protagonists can work... though my suggestion with it would probably be "don't drag it on for too long": the longer you spend fleshing out your "false protagonist", giving them a plot, an objective and a personality your readers get attached to, the more your audience is going to feel cheated once that character gets killed off. This is especially true if the entirety of the plot you had up until that point pretty much dies with the character too.
WARNING: A SONG OF ICE AND FIRE AND LAST OF US 1 SPOILERS AHEAD.
One of the things that REALLY irked me about ASoIaF, for example, was how Martin basically spent the entirety of the first book telling us the story of Ned Stark, how he made us follow his "investigations", had him discover a bunch of things, making readers attached to him... and then freaking killed him off. After 500+ pages. Without even giving him a chance to tell anybody what he had discovered or bring the plot forward, basically. Granted, that helped setting up the mood of the rest of the series ("people die, don't get attached to anyone"), but honestly, as a reader, I felt SO cheated on. It was like all those 500+ pages I'd just read were just useless. I picked up the second book, eventually, but... still, couldn't bring myself to finish it because at this point I don't trust him as a storyteller anymore.
A case of "false protagonist" which I think worked better was the Last of Us. I didn't play the game itself, but I saw the introduction on Youtube and I think they did a pretty good job with it: you've got two characters that you start getting attached to, the game is actually very good at making you think that the plot is gonna revolve around saving/protecting one character in particular... and then BOOM, that character is killed off and you're left in tears. It was shocking, it was jarring, it was sad... but in that case, as a part of the audience, I wasn't feeling "cheated", because it was still the very first part of the game and even though I was emotionally invested, the story was still at the very beginning and I knew that a lot more stuff was going to happen anyway.
In short: tread carefully, make sure your audience doesn't get too attached to your decoy protagonist (e.g. by dragging their story for too long) and, most importantly, if you have to kill them off, make their death meaningful in some way.