I am sure you didn't kill him at all - more likely he died from stress (wild birds are not too good at surviving in human care, except when in the hands of specialists). I suspect he had hit a power line or been attacked either by another bird species or another seagull (they can get a tad aggressive over food). I once rescued a young seagull from some horrible boys with sticks, wrapped him up in my jumper and took him home. He survived the night, ate a little bread soaked in milk (I had no idea what to give him) and I took him to the local PDSA as he seemed to have a broken wing. They told me about the stress bit and recommended that he was put to sleep because they were doubtful even they could bring him through.