Nah they're protected species in the UK. Idk what the law looks like elsewhere. It does seem a little cruel to punch the bird and it not die and then leave it to go wander off and die from its internal injuries. I'd have more respect it dude just broke its neck and have it a quick death.
Kind of reminds me of Canadian geese in the USA, I wish I could kill those bastards but they're protected. I don't know if it's possible to get a license to kill them or not, depends on how they're protected and the legislation around it - I'm too lazy to bother with any of that and don't hunt so wouldn't go through the hoops anyways. Those things are a menace and mean as hell a lot of times but you just live with it.
I'm curious now though if the USA has protections for seagulls or not.