Picking off AFKs is fine (they should be kicked for being AFK anyway). Catching people who spawn late and don't catch up to their team is fine. Killing someone who just spawned and doesn't even have a chance to swing their weapon is not fine.
If someone is circling spawn and couching players the second they appear, I would think that is an exploitation of game mechanics. I've never seen an admin kick a player for killing people as they spawn, though. When it is happening frequently enough to bother me I just go play on siege for a while. But I guess that means I am "letting them win" because I gave up and left. Instead of staying to play a videogame that is not fun. There is nothing more that I can write that some griefer isn't going to quote and reply "its ur fault u didnt spawn on time" and ignore the point I was trying to make.
I dunno, I can't really blame someone for enjoying spawn-slaughter. As an archer, my most satisfying shots are when I take down someone's horse then hit them with an arrow while they are defenseless, recovering from falling off the horse. You KNOW they are all "fuck, fuck" trying to pull up their shield while you place bodkins in their butt.