Why is being a murderhobo bad?

[Arbane]

A normal person would be traumatized by this, have some PTSD, but in DnD the LG paladin wouldn't bat an eye at the number of people they killed.

I shudder to think what sort of PTSD your average D&D character would have by level 5, let alone 20.

Because the D&D characters who are not murderhobos have backgrounds and goals beyond killing people and taking their stuff, and the D&D characters who are murderhobos do not.

For instance, my Ranger has a masterwork axe. This isn't his weapon, it's a tool. I do not expect the fact that it's masterwork to ever effect the game, but the two people I've known who actually lived alone in the woods were both proud of the quality of their axe, so it seemed in character. He also has a skill point in playing the harp, just because people who live alone often make music.

Murderhobo characters have no characteristics that aren't specifically aimed at killing folks and taking their stuff.

I think this might be getting to the meat of the matter - a Real Roleplayer(tm) tries to make things more complicated for their character, with social entaglements and hobbies and such. A Murderhobo tries to simplify it, so there's nothing to get in the way of the killing and taking of stuff.