Who Kills Thanos? - Predicting Avengers: Infinity War's Climax

12. Scarlet Witch

At this stage, Scarlet Witch is something of an MCU wild card: she's phenomenally powerful (even though she's been somewhat hamstrung by her mental condition) and as a walking, talking personification of an Infinity Stone, she has a slice of the powers Thanos will try to use himself. That spells poetic justice.

She doesn't quite have the personal link to Thanos to really justify this story development just yet, but when you consider that there's a distinct possibility that Thanos (or one of his Black Order at least) will kill Vision to remove the Mind Stone, that's all going to change.

With the death of her beloved, Scarlet Witch will be both unstable enough and hurt enough to really bring it to Thanos, and by the time Avengers 4 hits its crescendo, we could see her become the key player in his downfall.