Avada Kedavra is the least of the 3 unforgivable curses because it’s a painless death. Imperio is far more horrifying.
Yeah, it’s weird since there are many other ways to kill someone in Harry Potter that seem way worse but they’re not considered ‘unforgivable’
For example, look at how Molly Weasley killed Bellatrix and tell me that this isn’t also a “one way ticket to Azkaban”