Why? The vandalism that drove the victim out of the game was performed while that player was offline. Why should it be OK to steal and destroy when your victim is absent, but not to murder?
One is an attack on an inanimate object, the other is on a player controlled character. Player controlled.
Besides, griefers aren't role playing, whatever they may claim; they are acting out their normal real life personalities - getting their jollies by upsetting others, the more upset the better. It's an attack on the player, not the character, and should be responded to in kind.
You are confusing thieves with griefers.
You weren't here when griefers were working hard to make the game unplayable for everyone else - at least, you weren't here for what was very nearly their victory. The mechanics have made griefing somewhat more difficult, but only recently, and I'm sure there will be counters developed - indeed, I've seen new strategies already enacted. The devs don't choose to ban players for egregious grieving, which is what I've seen in many other games. It's excessive and immoral to hunt them down in real life and beat the stuffing out of them, not to mention illegal. Killing them while offline is a poor second to banning them, but I'm glad the devs at least give us that. And no, I have zero interest in role playing a man hunt - or more correctly a vermin extermination trip.
Once again confusing griefing with thievery, however the issue I take here is the fact that you claim you have zero interest in a man hunt. This is kind of the issue, it's the mindset that you believe you deserve to punish someone, you shouldn't have to work for it because what they have done is wrong on some kind of out of game moral level, at least that's what your use of language suggests. That's the problem, right now there is very little work required to kill someone if they have committed a crime, because they can be killed offline, because they are never safe once they commit a crime unless they turtle away somewhere, which is boring for all parties involved.
If it were just theft, and not too easy or safe to perform, I'd be less adamant. But it's not just theft, nor even mostly theft - at least in game terms. It's destroying other people's fun with no benefit to the in game criminal whatsoever - just benefit to the player, who apparently gets his or her rocks off by hurting and upsetting others.
Griefing again, but the game provides tools against this. Walls for example stop macroers in their tracks. Take the precautions and it won't be an issue.