Thanos spared the Watcher to show off his power, not out of kindness, confirming his true villainous nature. The Watcher's purpose is to observe everything without ego, contrasting with Thanos ...

Instead of destroying Earth and the Avengers, Thanos reaches the Realm of the Abstract Entities to stand trial. That’s where Uatu the Watcher will defend Thanos, seeing in him some hope to help ...