Don't feel bad, I killed at least seven trees this year. Probably closer to ten or twelve, depending on whether or not some of the stragglers decide they want to wake up. Just about all of 'em were due to me screwing up - too much water in autumn, too little water in winter, bad soil, too little protection, and some of them due to the brilliant decision to bring them in when they started waking up from the unseasonably warm February.
The real failing is in not learning what went wrong and figuring out how to address it so you don't repeat it.