Dad was strafing a barge on the Inland Sea once. As it passed under a bridge, he said he held the trigger down, figuring the fifties would either go thru the bridge or not. Post-mission, while watching the gun camera footage, he saw something he'd been concentrating on the target too hard to see at the time - a Japanese woman running across the bridge. His words: "Man, those fifties just blew her all to pieces. Felt kinda bad about that. But - oh well!" I got the impression he DID feel bad about it, but accepted it as "what's done is done."
I think my Grandad's biggest regret about WWII was that he didn't get to kill more Japanese. Although he was an avid hunter and trapper before going to war, but never hunted again afterwards.
It was 1985 before I got him to buy a Japanese car. Even then he kept saying it was hard to resist the urge to shoot at it.
My grandad on my mom's side was a Seabee who was pretty much permanently stationed in Burma doing massive construction projects such as the Burma Road, the Ledo Road, and airstrips supporting the Flying Tigers (after the AVG became the 23rd fighter group in the 14th Air Force). He was a pretty good engineer, but he had even a greater skill. He had the best slider in the East Asian theatre. When his division commander realized he was working on a base that was overrun by the Japanese in 1942, he got pulled from the front lines. There was limited supply in theater due to priorities elsewhere and Japan cutting off supply lines (like they overran the air base he was working on). But as usual with the US Army there was a huge black market. How did they determine supply priorities? By betting on baseball games. It was basically a form of legalized bribery, because the supply NCOs that actually ran things wanted to make money on the side, but Stilwell was shooting pilferers. So they'd send in my grandad as a ringer, get the clerks involved in the action, let them know the fix was in which made them happy, and get higher priorities for supplies. My grandad's slider helped save India from Japanese expansion. Or so the story goes.