Hello, I found this game by accident. I have played the first two Fallout games before. The MMO feature is interesting and I am sure to play this again.
I was already aware of thieves and player killers. So far I only got robbed once because I was AFK for a few minutes, silly me. XD
If I run away from you when you stand next to me, sorry, I am paranoid. 
That's what happens when I play Ultima Online for many years when free stealing and PKing was still around back then. 
Don't apologize, it's a good habit to have early in the game. IRL some people like their "three feet of space", translate that into "three hexes of space". Also, if you have anything valuable on you, put it in your hand slots. This will prevent it from being accessible to thieves.
Also of note, when you log out your character will stay in it's last position for 3 minutes before disappearing. During this time, everyone can see you are logged off, and you can be stolen from or killed during this time. So when you log off, do it in a tent or on the world map.
As far as PK'ing goes... it's gonna happen. It happens to everyone. Getting upset will NOT help. The only tip I can offer is that sometimes just being friendly will yield the best benefits (after the fact), even if you just had your head popped by a sniper rifle for no apparent reason. I know many players on my "no shoot" list, just because I like them. This won't stop it completely, but it'll narrow it down a bit (maybe). Raging only feeds it.
Also, some people might kill you for reasons not apparent to you, even if they aren't PK's. If you drop into an encounter with a player, and they are busy fighting critters, just standing there or advancing might make them feel you're a threat. They may shoot you then, even if they did not intend to. So best option there, is that if you don't intend to attack that person, it's best to show it by heading the other way as soon as you get the chance.
You CAN meet people this way, as well. But you're taking a risk, so as long as you're willing to lose what you have on you, go right ahead. It could make you friends. This is not advised if you're carrying several thousand caps, or all the ammo you just crafted to level with, etc.
Last thing - it's a game. It can be fun if you let it. It's also very frustrating at times, and can be hell if you let it.
See ya out there...
