Harry isn't the type to actually kill someone.
Harry, king of Expelliarmus, isn't the type to kill anyone if he can avoid it. He doesn't kill. He believes in doing as little harm as possible. When Ron asks him what to do with Thorfinn Rowle and Antonin Dolohov, who were both Death Eaters convicted of serious crimes, he rejects the idea of killing them.
“What are we going to do with them?’ Ron whispered to Harry through the dark; then, even more quietly, ‘Kill them? They’d kill us. They had a good go just now.’
Hermione shuddered and took a step backwards. Harry shook his head.
‘We just need to wipe their memories,’ said Harry. ‘It’s better like that, it’ll throw them off the scent. If we killed them, it’d be obvious we were here.’
‘You’re the boss,’ said Ron, sounding profoundly relieved.”
- Harry Potter and the Deathly Hallows, Chapter 9 (A Place to Hide)
When Lupin suggests he use more aggressive spells, he defiantly rejects the idea, and still prefers using spells that won’t cause lasting harm.
“Harry, the time for Disarming is past! These people are trying to capture and kill you! At least Stun if you aren’t prepared to kill!’
‘We were hundreds of feet up! Stan’s not himself, and if I Stunned him and he’d fallen he’d have died the same as if I’d used Avada Kedavra!”
- Harry Potter and the Deathly Hallows, Chapter 5 (Fallen Warrior)
Even in his final battle with the Dark Lord, his most dangerous and most deadly enemy, he uses Expelliarmus. The only reason he even tried to use Crucio was because he was upset that Sirius died. Even so, he wasn't able to summon up enough anger to cast an effective Cruciatus Curse.