I think it's the sentiment that people are railing against rather than the actual act, which no doubt took some dedication (to griefing). If they could have enticed a normal player out there to be slaughtered under some pretence, that would be infinitely more acceptable and more impressive. The fact that they took advantage of someone's kindness is unacceptable in my view. Kill someone that came to help you in good faith...that person took time out of their routine to do a job they don't get paid for, helping others. Takes a special type to do that to someone else in order to set a 'record', which isn't even valid cos they self destructed so technically didn't make it to a dock with the 'kill' (in order for it to be confirmed, let's say).
If someone crosses a mountain range on foot, then pretends to be stuck, calls mountain rescue and then murders the rescue team, do you let them off cos of the effort they put in? As others have said, it's just a game, so that's not 100% relevant, but it's an accurate analogy of the sentiment. This act crossed a line for many people, myself included. I know it takes all sorts, and we all have to deal with it every day in the game and reality. A spade is still a spade though, and I'll call it one.