Keith was on a mission with the Blades to take down a Zaiforge cannon when things began to go bad, Voltron in danger and a Galra ship in the area seemed to be the issue. So he departed on an extra Galra fighter and met up with the rebels to try and stop the witch from destroying Voltron.
Later in the battle realization sunk in on how this battle would have to end with a sacrifice, he thought of who it would be when he realized it had to be him; but why should he care? Keith thought to himself on how team Voltron had pushed him away and the things they had said to him-
Stop being selfish, you're not important to the blades, nobody cares, just leave already.
The thoughts replayed in his over and over and that is when he made his decision, in a barely audible whisper he said
"Maybe it takes more than guns to break down the forcefield."
"What are you saying Keith?" Matt responded in a hurried tone.
But he never got a response as Keith switched his communications to mute and began to fly towards the barrier. Keith could hear Matt screaming his name telling him to turn back on his communications while also frantically trying to reach Voltron to have them stop Keith. It was too late, Keith prepared for impact while he suddenly wished it didn't have to end like this; although his team had neglected him Keith couldn't help but remember all the good times they had. Unfortunately there was almost no time as Keith looked ahead into the blinding purple lights he closed his eyes and took his last breath. As Keith's ship hit the force field there was a wash of electricity that rushed over the Galra ship and allowed the explosion on Naxzela to be stopped and Voltron to be saved. The relieved and happy voices of the Paladins of Voltron, the Blade of Marmora, and the Rebels flooded through Matt's speakers on his ship. But all he could do was look forward in horror at where the former Black and Red Paladin once was but now there was only bits of debris floating out into the void of space. Matt then took a deep breath and ordered his Rebel squad to go back to the castle, he had some heart wrenching news to share but he didnt know how yet.