How did he kill most of the resistance?Finn and Rose didn't alter Holdo's plan. Her plan happened right on time.
Easy. Remember the scene where Finn, Rose, and the codebreaker are nearly in to the tracking room? Finn gets on the radio. Poe communicates to them on that radio that the ship is sending cloaked shuttles to the outpost. Until that point the shuttles were hidden and that was unknown. Codebreaker sells them out and throws in that information about the cloaked shuttles. That's why Finn and Rose are forced to watch them be destroyed. They killed those people. Had the shuttles remained cloaked it would have just been Holdo taking fire with the rest escaping. And that would have happened if Poe was not revealing their tactics openly.
The codebreaker already ratted them out though. They were ratted out before they got to the tracking room. Which is why there was a trap there. He didn't even rat out BB-8. He was just stopping them from disabling the tracker. I don't remember how they were able to see the ships but i don't think it was the codebreaker who did it. but then again, i probably have to watch it again. Even then though if that is what happened, Poe's mission would have been more succesful then holdo's either way. Cause all the ships would have been saved and not just the escapees. That should have been a last stand scenario. But Poe didn't trust Holdo and she didn't trust him, especially since he was just demoted.
You're misremembering.
They weren't ratted out at the start - the evil BB unit spotted BB-8 hiding under the crate and put two and two together (which is why he's there when the troops arrive to ambush Finn and Rose). After they get captured, DJ offers up information on the cloaked transports in exchange for his freedom and a payout. As Finn and Rose are in the hangar bay waiting to be executed, an officer runs up and says that they ran a scan for cloaked ships and found the transports, just as DJ said.
DJ wasn't evil - just in it for himself. When his job for the Resistance went pear-shaped, he switched sides. ---
Kill 1 man: You are a murderer. Kill 10 men: You are a monster. Kill 100 men: You are a hero. Kill 10,000 men, you are a conqueror!