Spider-Man in No Way Home. The Green Goblin almost caused him to become a murderer and cross a line he would never come close to crossing because the Goblin killed his only remaining family (Aunt May).

Hawkeye from the MCU went on a killing spree after the Snap, killing many criminals that he thought didn't deserve to live out of "justice" to the many innocent that died because of Thanos.

Dinah Drake in season six of Arrow.

Masa Shimamoto had he not've been talked out of killing Lucks he'd have became a villain (OK I know he gets brainwashed into being one but that doesn't count because it's brainwashing and not his fault)