It's crazy how much the ending credits put things into perspective. The people in the AoT world might be even more fucked up than the people in our world. lol
It's hard to say whether or not Erin was right or wrong for doing what he did. Obviously, killing 80% of the human race was terrible, but his head was fucked up. Based on what he said, he couldn't see anything other than that outcome. He didn't really see that he had a choice. It's a crazier version of what Thanos experienced in the MCU. With Thanos he studied different societies and came to the conclusion that population control was the best course of action. With Eren, his believe wasn't just supported by a few hundreds of years of science experiments, he could actually see the future.