Board Thread:Questions and Answers/@comment-39492511-20190519172422/@comment-37493549-20190520010341

It was impossible to allow him to live. He could transform into the Colossus Titan at any moment, killing everyone around him in the explosion.

The Colossus Titan was also far too valuable not to acquire. It was probably capable of single handedly wiping out every pure titan on Paradis by itself, even without Eren's help.