First all, you need to pay attention to the various race and class's that are out there. An evil duergar anti-paladin might not need much motivation other than pleasure for loppin off your head. Carrion fields is very dynamic though and you can roleplay any possible reason for someone to kill you.

You could be a goodie sitting in town as a Tribunal, and an outlander elf might attack you. There are so many diferent reasons and as long as you are rollplaying a char it is acceptable. If you for example rolled up a gnome which is supposed to be a wise and balanced race...and started attacking everything in sight, you will be punished by the IMMS. Same thing if you are just attacking out of character. You will be punished harshly.

You need decide when you roll up a char, what type of char you want to play. Evil bastards get way more flexibility in whom they can attack, because welll...murder is evil.

As far as killing someone again after you have killed them...its all fair game. The game is designed so you have 15 real life minutes to go gather some more armor and get the hell away from who ever is killing you. As you said, the game is huge, so if you plan to stick around and talk to the murderer, be expected to be gutted once again when you are corporal.

Best thing is to try play a neutral type char that is usefull to good and evil. A bard would play that role. Goodies wont be out hunting you. Some evils will. Other evils might ask for your aid while gathering, exploring...

Best thing is to write up a role and stick with it.

Hope is the worst of evils, for it prolongs the torment of man.